So the newer versions from K2 are oil-filled? I wonder why they switched to that, cost optimization?
Regarding the boring, believe it or not, I just read the respective section in your book last night 
Yeah, it has to be rebored, I`m convinced. On ebay, the piston set from Japan from `cruizinimage` seems reasonable to invest into, have you worked with those? I think I used their set for my Yamaha twin and so far they are fine. If I go with the big-bore kit, what else has to be changed apart from the kit? Rejetting is also needed I assume? I appreciate your input!
Those shocks might be tough to rebuild: Honda didn't intend for that. In those days, though, a pair of Showa gas-charged shocks was only $26 out the door(!).
A comment about the boring: please see the section in my book about it. The cylinders WILL need reboring now, if they were still original size. The metals warped around the pistons, making the holes several shapes (except round) and re-ringing the stock pistons usually only provides 8k - 10k miles of reasonable (not increased) performance. A step-bore of even 0.25mm (1st oversize) will bring back all of the original power and some more, and it will stay forever because the bores are cured and the holes will now stay round.
This is true of all of these engines (singles, twins, Fours) or the era, not just Honda's versions. I strongly recommend at least the 0.25mm overbore, but be DEAD SURE the machine shop bores them with the specs shown in the book: piston clearance of 0.0008"-0.0012" is REQUIRED, not optional. I rebuilt my own 750 at 138,000 miles to 0.0006" clearance, and the difference in power and smoothness over the previous rebore (55k miles) at 0.0012" clearance is amazing.
I often use the CruzinImage pistons in their 0.25, 0.5, and 1.00mm overbore sizes. Once in all of them (well over 100 pistons by now) I got one that was a little malformed near the piston-pin boss, and I wrote to him about it: a week alter I got 2 new pistons, no charge(!). One was standard size and the other was 0.25mm oversize, he just 'wanted to be sure I have the right one'. I still have the odd one around here somewhere.
I don't have supporting evidence about the change from nitrogen-filled to the simpler oil-filled shocks, but I know that all of the early ones lost their nitrogen in less than 1 year. I suspect Honda didn't like that vendor after that, and changed parts. That was also when the shocks changed from 12.9" long to 13.1" long. The same springs seemed to be used, which gave the K1 New Factory bike a slightly softer ride than the K0.
The chain story is even more fascinating: by 1970 Honda knew they had a big problem with the chain, and RK Japan (the supplier) had no ideas. At the same time as the 750, Harley was prototyping their 1000cc Sportster in their older 883cc Sportser frame, and this new V-twin was eating the chain alive, too. Diamond made the H-D chains, and they came up with the XD chain for the bike, which had extra clearance in the now-hardened rollers and quad staking on the link pins, with shot-peened side plates. The Harley frame tended to fold up sideways under heavy throttle grabs (and sprang back like a spring during shifts), so their new arrangement let the power be transferred thru the chain without grief. Honda got REAL interested, but Diamond would not talk with them about a competitor's bike. About that time, H-D went bankrupt, too, and AMF bought them: Honda then quietly bought 51% of AMF stocks in early 1970. By March the Japanese bosses had nabbed the Diamond design from the Harley engineering group and took it to RK, who made their version of it (except they could not replicate the Diamond master link, so the new chains became endless type instead) while Honda's engineers also made the sprockets sport a larger (by 0.5mm) base circle, later 1mm more, to reduce the chain's crash into the sprockets upon entry. This also caused the rollers to pump the oil on the sideplates into the rollers at each exit from a sprocket tooth, and the RK chains could later over a year.
Meanwhile, we 750 owners found the XD chains (at racetracks, mostly) and put them to work: they outlasted the RK chains almost 10-fold in practice because of superior American metallurgy (in the 1970s), and I experienced many, many of these chains passing 40k miles on the original sprockets on bikes I serviced then. I still have my first XD chain, as it was stretched just 1% (one link length) at 40k miles when I changed off my first pair of sprockets of my K2.
On the dynos, the XD chains showed a clear superiority, too, so they soon became the dragracer's choice. My shop sold a LOT of XD chains. In about 1990 Diamond changed the name to XDL chain and made the XDL Powersport version as well, which is made for more cold-weather work (snowmobiles, etc.), but it works almost as well. I won't run any other chain, myself!
