Here's all the in-engine differences:
1. the front cam chain slipper tensioner is shorter at the top by 2cm length, because the (see#2)
2. heads are made from more accurately machined injection molds. (The heads before K5 were sand molds).
---------
To use a common slipper tensioner in the later heads, you have to file back the lip in the cam chain tunnel about 1/8" to allow the longer tensioner to stick up through the hole.
---------
3. The primary-drive ratio is slightly lower in the engines made after 1975 (F0/1/2/3, K7/8). This caused some changed gears in the tranny.
4. The clutch is slightly different from the early bikes (before K6) in that a slipper plate is added to the top of the stack, and in the stack elsewhere there are 2 steel plates riveted together with tiny springs between them, to let this special plate slip momentarily if someone drops the clutch too hard. This reduced the friction plates from 7 to 6. This was done to protect the needle bearings in the clutch hub where they ride on the mainshaft, to keep them from being momentarily overloaded. Not that I've ever actually SEEN one get overloaded, until Russ Collins got into the act...
--------
About the clutch: the later engines mostly (not all) have a clutch cover (on the right side of the engine) with clearanced ribs on the inside, so as to clear the slightly longer clutch basket with its special top slipper plate. If you put an early clutch cover on these later engines, the engine will not turn because the mainshaft gets locked to the clutch cover. But, you can put a later cover on the earlier engines (not counting the K0, here...).
-------
5. The lower cases are beefier in the output shaft area on the later engines. The other parts are the same in the area, but starting with the late K6/F0 the chain oiler was completely removed and the hollow final shaft was plugged: in the later bikes with the 630-size chains, this shaft became solid in most engines. The oil seal on this 630-size shaft also changed with the chain size.
6. The bikes made after 1976 have a spacer on the mainshaft, opposite the clutch side, that was selected at manufacture to make a certain spacing happen between the 5th gear and the clutch needle bearing. If this transmission is then plunked into another case, this spacing needs to be checked to make sure it stays the same, or else 4th gear can 'disappear' sometimes when shifting, making a false Neutral.
7. In the engines with the "392" number cast on the top flange of the head in between intake spigots 2-3, the pistons have a raised center section with a 3mm lower lip all around the edges of the crown. This, pushed into the same-size combustion chamber as the earlier -300- heads, makes compression slightly higher at 9.2:1 instead of the 9:1 found in (most of) the early engines. This was done because of (#8)...
8. The cams on all of the post-1975 engines open the intake valves at 0 degrees TDC while leaving it open the same number of degrees as the early engines. This allowed more scavenging and pushed the RPM powerband up about 1200-1500 RPM so the power starts at 5500 RPM instead of 4000-ish RPM. The needles in the '7a' carbs became thicker in the lower 1/8 throttle range to lean out this portion of the mixture. This was done largely to meet emissions rules of the era because the (dumb) DOT required the emissions be tested at 2500 RPM...a side effect of this change was: the sparkplugs stay cleaner, longer!
9. Beginning in the F2/K7, the carbs changed (with the airbox and all the hoses) to the more precise PD4x series, to meet emissions rules by restricting the user's easy access to things like float levels and jetting changes. Thank you, EPA...the first versions of these were the "086n" series of the early carbs, which have extended tips on the air screws to accept little black limiter caps (which mostly disappeared by themselves since then) so that you could not adjust the screws very far -unless you first removed the caps, adjusted the screws, then replaced the caps...
10. Beginning in the F0 bike, there is a little oil-air separator gadget under the battery that sucks crankcase fumes out of the back of the engine and dumps them on the ground: it was supposed to put them into the airbox to burn the fumes (like a PCV system in a car), but there is not enough suction in the airbox to make this happen. But, it did meet the [stupid] DOT rules of the era. These filters are usually now plugged up and do nothing, which makes the engine run better, so leave it alone...
Last, and of special note: the F2/3 bikes have longer valve springs and longer clutch springs. The rates are still the same as the earlier springs. The valves were changed to thinner intake valve faces (and slightly wider intake ports) to let a bit more air into the larger combustion chambers of these 2 bikes, and the domes of their pistons are high and rounded, resulting in 9:1 compression: these heads are identified with a "410" cast in the same place where "392" used to appear. This causes trouble if you don't get the right kind of pistons (410 type) during a rebuild, because of the large chambers - it results in low compression, like 8.4:1 in some cases. These valves also have a real bad habit of pulling their lighter, ridge-grip retainers thru the top spring retainer, dropping the valve on the piston at 8000 RPM and raising Cain.
All of the Honda cams from the K6 thru the F3 have steeper closing ramps for the intake valves. This was first done in the late K5 to (finally) stop the plug-fouling problems many of the bikes suffered when owners simply refused to run the engines hard enough to keep the plug clean. It also resulted in bikes that would not go faster than 98 MPH, until the F0 design changed everything else. If you have an unknown cam in your hand, look at the closing sides of the intake lobes: if they still show their Parkerized finish for about 3-5mm on that surface, it is likely a K5-6 or F0/1 cam. If the lift is more than 7.4mm on the intake lobes, it is probably either a K0 or post-1975 cam. If the lift on the intake lobes is almost 8mm (7.82+) it is an F2/3 cam.
That's about it!
