parts for MC21 are largely available.

parts for MC18 are harder to come by. cranks for 1988 version are no longer available. cranks for 1989 version can use a modified MC21.

pistons and barrels are available for both.

don't let a wire splice or holes drilled in air box scare you. government regulations only allowed 45 HP for this "beginner" bike. HP was reduced via ignition timing to comply. Almost all of these bikes had this restriction removed to gain extra HP via a simple wiring splice.

additional air flow also gains some HP. tried and true, four 1" holes do the trick.

you can learn all the details about both right here. learn to use the search feature.
_________________
MC21R9N track
MC18R2J track
CBR954
CR250