The first run had 2-ring pistons and IIRC the rings were 3/16" thick. Next run also had 2-ring pistons with 5/64" thick rings. You have to go by serial # with these. Probably the switch to a 3-ring piston was with the '65 models but don't hold me to that! At any rate, the bore size is the same and you could use 3-ring pistons in that powerhead as well.
The rod size on the Big End of all the 90 c.i. engines is the same, so you could use whatever piston/rod combination you find.