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Pretty sure the landings would show up on the richter scale, braver than me

I seem to remember that rods are impossible to find for the RM500 which is a major part of why you see so few of them being raced in vintage mx, hopefully the motor is sound

Assembled up this bottom end, drilled the exhaust bridge lube holes & fitted the cylinder. Waiting for the viton o-ring for the custom crankseal holder to finish it off. Still to decide on stock or period aftermarket head

In theory they should respond well due to being 1 1/2 T500s with watercooling, both were tuned more for a wide powerspread rather than peak power
I had a friend build 1 up for post classic racing using a ex speedway sidecar motor, when the pipes were being designed with 1 of the programs they found they needed to add transfer to give a better powerspread. I do remember the pipes that were built were run the same as the TR750 with 1 curling around behind the block
From memory the 500s & 750s don't tolerate spending their life in the redline, the skirts are known to crack as the left 1 appears to have done if over revved to often
Should be an interesting project

Had the same problem with my TY250C except someone had cut the stump of the cable off, never did manage to get it out or unscrew the retainer fro the worm gear. Heat was never a option in that case due to it being magnesium. We just ended up blanking off the hole see as the speedo wasn't needed & the inner part had already been removed by a previous owner
A mixture I have used for unseizing motors that have sat for decades is 50/50 kerosene & brake fluid, seems to break bond without damaging any of the metals. Stand the plate up & fill the hole with the mixture, leave it a few days to soak before seeing if it frees up

This is another of the interesting TZ750 stories from back in the day, could you imagine Yamaha deciding to try endurance racing a M1
https://translate.google.by/translate?hl=en&sl=es&u=https://vivelendurance.wordpress.com/2017/09/05/yamaha-tz750-ow-31-bol-dor-sonauto/&prev=search

Strange, guessing whoever did it built 2 engines up at the same time.
Knife edging isn't something you see done much anymore, saw some photos of the last GP bikes ports & the dividers were quite thick & rounded instead of sharp
What are your plans, 2 cylinders the same or close your eyes & reassemble?

Don't know why you didn't see these on eBay, may be you looked for PWK28s without knowing most of the 80/85s are fitted with them standard
https://www.ebay.com/itm/2001-2013-Kawasaki-KX100-MX-Complete-Carburteor-Assembly-15003-1643-OEM/252679553156?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

I raced a YZ465H for a few years with vapour blasted top end & found that there was no staining or corrosion even when I sold it. I definitely prefer it to the original painted surface & did the same thing to my TY250C, makes it bike look a bit more worksy to mind as well

Sounds like a heap of fun

Even the OKOs have knock offs now, Mid-Atlantic Trials did a good write up on how to pick the fakes & what the actual differences are

Colours back then were so much more interesting. Both my T500III & GT550J were the boring blue but being a candy colour they looked good, much better than the plain colours & decals used now

They look incomplete to me, knock the bridge between the holes out & they would be more like what I would expect

To get the best effect of the reed upgrade you really need to remove the step. Do it before you give  the cylinders the wash to get rid of any grit left after honing, I had already had my cylinder bored & honed before modifying the port

Try to blend out the step where your uncle finished machining using either a die grinder/dremel or riffler files.
I fitted the YZ85 reeds into a TY250 after reading about which block to buy on the old forum - RDs & TYs have the same reed block going off part numbers.
I was able to use my dremel with ballnose endmill to remove metal quickly which takes a steady hand, I had marked how far into the cylinder the reed needed to sit before I started & knew how much needed to come out at the deepest point & kept trying the reed block in the hole until it fitted nicely. After that I just tapered the port wall out until it met the liner & knocked the lumps off the casting but didn't touch anything with sandpaper
Takes a bit of time but is better than wrecking a cylinder by breaking out the side of the port

This is how the sidewall looked when I'd finished