Well, yes but you could fit the cylinder off an XL600 or NX/XRL 650 and get a piston for those. I don’t know if the pin diameters or deck heights match, but surely you could make a custom conrod and/or piston. Agreed, it won’t be off the shelf items, but with this kind of experimentations and modifications you wouldn’t fit stock items anyway.
Yes again, but for any given engine the smallest stroke is a recipe for rpm and the biggest bore will give you the extra capacity for additional power (well... torque actually). The result will also depend on the weight of the moving parts and, as you say, compression, cam, valves, porting and the lot. Granted, there will be compromises somewhere along the line but I’m pretty sure you can’t get any better for this series of engines.
The bottom line is that going this way should result in a screamer old-school engine, or as close to it as a 25-year old design can get. (Alas, I believe that the hardest part would be keeping the engine together!!!)
I’d VERY much like to see the results of your project motor!!! When do you plan to start?
Sect#1 Pin diameters are different and the cases need to be bored to fit the XL cylinder to 500 bottom end. 500 crank may be lighter but not that important. No point to using the XR500 anyway, XL600 motors are far more common and have the larger pin to take the extra power. Sect#2 Yes oversquare is the way to rpm but the head is just as important. While the RFVC heads look like a great design they actually are self limiting. Valve sizes are limited because they will interfere with each other at higher lift and duration. Rocker arms and sub rockers with screw adjusters make rather heavy reciprocating mass compared to superbike overhead cam and bucket/shim setups limiting rpm. Not to mention the mass of a 4" piston and rod and 3" stroke. That's equivalent to the b/s of a Z28 302 Chevy or Boss 302 Ford which peak at about 7000 with cam profiles similar to what you can run in a thumper motor. You can't run a cam as radical as a V8 is capable of running because of vacuum issues. Unless you want to push start and have 2000 rpm or higher idle speeds like the mile dirt track bikes. Sect#3 I already have the XL600 with 102.4mm, 10.1-1 CR. That's 617cc. Unfortunately it only has the stock XL600 cam in a lightly ported XR500 small chamber head. It also has the smallest XR flywheel I've ever found. From an early 83 XR500, 1/8" smaller than late 83/84 or all 600s. It still runs great and does rev pretty well. It definitely revs quicker and higher than an XR600 and makes the power more from midrange up instead of the bottom end torque. The power is more similar to a CRF450 but with much more low end and at least 10 hp less. I have Ferrea SS 1mm oversize exhausts, Kibblewhite SS intakes(couldn't find 1mm overs), springs and Ti retainers, shortened bronze guides, and an early HRC RS600 cam to go in. I have to put together the 641cc motor with WB#1600 cam for the bike before I can tear down the 617. It would be a bit of a waste to put all that good stuff in without porting the head so it will be a while before it happens. I have "Desktop Dyno" computer program that predicts the 617 should make 48-55 rwhp. More important than the hp is the fact that it should rev nicely if not insanely higher as you say. It will be as much like a superbike motor as is possible for a thumper to be. I'm sure I'll like it but I don't want or expect 10,000 rpm from it. I'll pm you when it happens, don't hold your breath. P.S. I've got a new spare set of RD#1010 springs if you're interested in collecting parts for your future revver.