The cheapest I've seen are from BoyGoesFast on ebay, cylinder, piston, and rings for $25 + 10 or $15 for shipping, He might cut you a deal if you buy all of them at once. His cranks are a decent price as well.
Cylinder + Piston set...
Browse his other items for the crank, he should have them for about $25 as well, Not all 66/80cc cranks were created equally tho, some are 38mm stroke and others are 40mm, with the 40mm ones becoming more of the norm, but still worth asking before buying.
I've seen bare crank cases as cheap as $15 but can't find the link... I think the going rate for a set of bare cases is around $30 but definitely worthwhile to shop around or ask for bulk discounts etc.
can you tell I'm interested in your project as well?