If amazon doesn't take the jug, I'll give you $30 for that crap set up ! I need some cheap stuff to experiment on!
65.94mm piston is the correct size for a 66mm bore. The extra is for heat expansion. You'll also notice the skirt is wider than the crown. Pistons are oval, not circle when cold!
If you looking for something reliable, get a hold of ken and ask for a cast piston of his recommendation. Forged is nice if your pushing over 9k RPM, otherwise it's sort of a waste. The forged is stronger, but doesn't show it's colors until you exceed that limit. The inherit forces of the piston accelerating and decelerating so fast will literally rip them apart. It's one of the limitations of all engines. For the blaster, we're looking at 9375 rpm for a cast piston, and 10700 rpm on a forged.
Your rusted cylinder may not be to horribly bad. I had 1 that looked amazingly bad. Cleaned it up, it measured within spec, ran a stock piston.. Still running today a year later (not ridden much). Get a picture of that old jug and if you can. Measure it. It could be stock, it could not. If it's stock, you can run that piston you got.