Just looking at the size of the decomp hole in the original bore compared to the re-sleeve i'm thinking there's scope for enlarging it a bit to soften the kick through. Original hole looks like its way larger than normal but might be just my computer screen & i can't save & expand the pics for some reason. Lower exhaust port will give it more compression pressure to kick also--"i think?" but might be wrong. I guess whatever size hole--the hole to the exhaust outlet from the valve would need to be equal to benefit from a larger hole, plus the valve ability to flow through as well. Never mind lol--too much information. Oh, & it shouldn't kick back in theory due to the retard part of the trigger on this original type. But--they used to sneeze while kicking from not producing enough voltage at kick speed & starting was a bit of a lottery. I heard of owners getting the source coil rewound to a higher value to fix it.
Ignore me i'm just reminiscing over old times haha.

I am not an engine builder or tuner, just an enduro fan concerned about rare SC500 pistons.

One thing you might consider is that by lowering the exhaust port you are effectively raising the compression ratio.  Relieving the combustion chamber or stacking gaskets could reduce the CR if you find it too high.

Run her on av gas & might be a super grunter.

Hell fire bagsy not kicking it up.
Interesting to see and read about the reeds and beefing up the source coil.
Just to prove how stupid Yamaha can be regarding fuel supply methods i would say take a look at the RZ 500 V4 .
Idiotic and my autistic son could have designed better!
No wonder it couldn't live with the Suzuki RG 500 square 4 of the same era. Suzuki had 4 carbs on rotary disc valves .
Yamaha on it's V4 had  2 crank case induction and 2 cylinder induction both through reeds. How stupid !
It just didn't work . So i think you're on the money . Can't be any worse !

I agree on the decomp hole. Might be a little small. I considered opening it up but I think I'll try it as is. It will help for sure, and it only needs to be big enough to bleed off compression at kicking RPM. I don't know what was going on with the original bleed hole. It is gigantic. No telling if it had been modified, these are old parts of unknown history. I assume they came that large from the factory.

This motor will run the Rex's Speed Shop full race ignition I have on my 360 currently. Should be a lot better than the original components.

By my math the 3mm lower exhaust port traps an additional 21.25 cc's of volume.  About 8.6% more.  The service manual states the combustion chamber volume is 57.5cc's not accounting for the dome of the piston.

I can't find published figures for compression ratio but if it is 7:1 it would bump to about 7.6:1.  With the advance/retard feature of the Rex's kit, that might be just fine.  On the other hand, I haven't verified that my kit is actually doing that.  I read recently that a member on here had observed their kit operating with fixed timing.  I will put a timing light on my 360 one of these days to see what it's actually doing.

Just to be safe, probably depending on what the cranking compression looks like, I would remove some material from inside the combustion chamber dome to lower compression to a safe level.  I've got a finely tuned ear for detonation so I'm not too worried about blowing this thing apart.  I hope to get it right on the first go around though.

The Sc500 I’ve own started easy if timing is correct and decompression valve is working. 

Is the Rex ignition the full flywheel type not internal rotor, as you'll need the inertia of the flywheel to help it follow through with the kick, even more so with the lower port--i'm guessing. Have a 6mm higher exhaust port on my RT2 cylinder & starts ok with a good kick & no decomp with a points mag & high comp head. Mind you, i'm only dealing with a toy 80mm not damn near 100 of a 500cc mans bike haha. Re- the decomp, i'd measure the size of the hole leading down to the outlet from the valve as i guess the cylinder hole should match that from new. Is angled slightly to try drills in from the bottom up to find the size--& they block up on AG175 farm bike but they had them open all the time & steep angle drilled from the top. On my YZ465G i drilled a hole like the AG bike as enduro used it for a bit after i'd finished MXing it & softened the almost idle running through trees without stalling plus easier to start. Anyway--bit of useless information. RD400's had that hole too, open all the time.

The thing not to forget is the screwy way the Japanese decided to measure compression ratio on the two strokes.  7:1 is likely around 12 or 12.5:1 using the conventional way to measure compression ratio.  These are not low compression engines.