I wonder if keeping it at 736cc, finding lighter pistons and such,increasing valve size, heavy duty rods, and good headwork could get you safely (and still be making power) at 12k rpm. Proper cam would be needed, and lots of low end would be gone, but you'd have more of a modern bike feel witht he engine (rev rev rev, basically).
Ah well, sounds expensive.
It was expensive circa 1974, and a little work.
With the transmission attached, 14k is possible, but not real streetable. Removing the trans (for 1/4 midget racers) lets them go 16k. I even got one with a trans to go that high, but it was pretty flat from 14k-16k, and it required running the oil pump from the end of the camshaft with a 1:1.6 or 1:1.7 ratio up-gear box and a different pump, with plumbing (into the main journal on the right side of the engine).
That said, 12k is streetable, if you're not touring with a friend, because the power band is narrow and low-end torque is reduced. First thing: buy a new oil pump. They wear pretty quickly into an 80% flow situation, like in 5k miles. Next, cam timing needs to be wider and later than you might think, because the engine is undersquare. Spark needs to come up later, or you can't leave a stop sign, then it needs to go further, like 45 degrees. The carbs must be polished, the intake tracts cleaned up and a pocket carved over the top of the intake and exhaust valves. Velocity stacks on the carbs are a must: use the longest ones you can fit into the frame. These provide a BIG improvement at (engine) speed.
I did it with transistorized ignition, triggered from Hitachi points with doubled springs, and the spark advancer's open/close ramp was reduced a little, while the open portion was welded up and smoothed to make sure they opened sufficiently. The condensers were hand-selected, as they vary about 20% intolerance: all this trickery might be avoidable with today's electronic ignition, but I haven't tried it, yet...
The cam should be longer duration and a little more lift (mine was only .033" over stock lift, +28 degrees, I think), but not like a dragster cam, more like a hot street cam, then set about 5 degrees late. Light spring keepers and stronger springs are needed: I used Yosh double springs, but I think MRieck has the dope on today's versions more than I have, at present. Yosh's are unobtainium today: they were roadrace springs, developed after Mann's famous Daytona ride. I don't know what the material was, but they were as light as plastic, and as expensive as gold! Get new (or steel) cam chain rollers and slipper tensioner. Try to find a heavy-duty cam chain, the non-master-link kind. Hand-smooth the edges of every tooth on the cam gear for smooth entry-exit of the chain.
Match all the intake tract (hoses, etc.). Spend so much time with the intake tract that you see it in your sleep. The K0-K2 heads flow better than the K3-later heads, probably because the holes are bigger. Polish the valves. Then polish the intakes again. Then polish the guides and the stems where they stroke in & out of the guides at the business end. Remove the valve seals. Make sure the valve clearance is .0008", no more than .001", or replace the guides (I would, anyway, for a roadrace-type setup). See if someone can install bronze guide inserts for you: tricky, but not impossible on these. They REALLY reduce valve stem friction.
Pistons: cut off the skirt, right up to the rib. Then bevel and round the bottom edge. (Today, someone might make shorter pistons, I'm not sure - but that would help.) Then, using a sharp scribe, make some vertical "scratch" marks in the piston skirts up to the area where the piston narrows down toward the wrist pin. Do this approximately 1/4" apart, all the way around the piston, not deeply, just until you see new metal. It should be about .005" deep maximum. This is tricky and time-consuming, but needed to keep the pistons from burning. At the bottom of each scratch, take a triangular file and smoothly make a notch around the edge of the rib, about .010" deep or so. Smooth the edges of all these scratches and cuts with 800 grit emery or cutting Scotchbrite. Piston clearance should be .0008" to .001". Use a 60 degree crosshatch in the bores. Get the one-piece oil rings and carefully smooth out the sharp edges on the ribs that connect the upper and lower "halves" of those rings, to speed the oil removal during downstroke. If you study the rings, you'll see how this works, and you can see what I'm getting at from there.
Get new piston circlips. Get new piston pins. Polish the piston pins if they drag at all in the pistons with 10w30 oil applied. Get stronger rods and rod bolts (although 12k can be done with stock rods and stronger bolts, if the rods are good and the bearings are between .0008" and .001" clearance). Bevel the oil hole(s) on the small end of the rod(s) to improve oil flow: it runs behind a little bit, above 10k RPM, because of splash limitations.
Install a windage tray. You'll have to make your own: Yosh got $105 in 1974 dollars for mine (that still hurts). Cut slots in it, or louver it with the vents opening away from the crank so that it "scrapes" the flying oil away. Clearance should be about .125" to .150" or so.
This part is hard to describe, even harder to do: on all my 14k+ engines, I made every intake tract the same volume as the others. The method is almost unbelievable (I described it to RXmanGriff once, but it took a LONG letter...), but the results are helpful. This balances every power stroke.
You might wish to consider a gearbox with a low 1st and close-ratio 2-5 gears. Yosh made them, so they might still be around, here and there. It really helped get things started on the street, and Hell itself broke forth when 2nd was found: mine went 92 MPH in 2nd at 14k with a 16-tooth countersprocket, 48-tooth rear and 4.00-18 tire. Most street races were over at that point...
That's all it takes!
