Yep, start with the 4mil crank and bore stock cylinders to the next size piston and have cylinders ported for extra stroke. coolhead with domes is nice mod also. It will allow you to play around with compression.
You seem to be getting spark, are you sure you have no leaks at intake. Is choke tube installed and in good condition. Try removing filters, lift slides and squirt some gas through carbs, then try to start.
you don't know if the cylinders are ported for the extra stroke. That's why the guys are telling you to send it to a builder. You can port cylinders for stock stroke, spacer plate, 4mill no plate, 5mil etc.
Did the other 5mil run great? do you have same pistons and rods? Did the other 5mil run spacer plate? If the other 5 mil ran great, then you need the same set-up. We have no way of knowing your port timing. Most guys prefer cylinders ported not to require spacer plate.
You can add a back seat to the Can-am 800r and some model 1000. The can-am has a larger bed. I think bumper to bumper is 9'6" You can put it in a 12' garage and stand two banshee on grab bar. Pain in the ass, butttt works
Brother,you got a lot going on. Yes 120lbs compression is good. Stock new is 125lbs. Prolly should try to fix one thing at a time. Are you saying the threads for the spark plug are stripped out in the dome?
I think he means drag ported cylinders.lol Do you have a cool head? Check your compression. With a cool head, you can change dome sizes to increase or decrease compression. Increases in compression can require race fuel and will bump up your HP. A properly tuned engine is the key.