Re: Recommend an 80cc motor
That's what I did on my last build... I used a Dax Gen IV bottom end with my old jug and piston since it was already ported and not yet broken in. You could use the Dax bottom end with the BoygoesFast jug and piston and come out ahead since BoyGoesFast sells the jug and piston kit for under $25 you would come out ahead, You'd need to buy a wrist pin and clips from one of the sellers tho but you can't beat $25 or less for a jug, piston, and rings, with an intake manifold bolted on.
For the head, the Puch heads bolt right on and cost about $40 and they're far better than stock when it comes to cooling and chamber design, and trhe Fred heads are around $65 or so but also offer superior cooling with excellent compression and a good chamber design as well. I'm running the Fred head on mine and I really like how well it keeps the top end cool, it's the better looking head since it's built for this engine, but the Puch heads are right at home and look pretty good on these as well, performancewise, they're pretty much identical so it's a matter of how you want yours to look. The stock heads can be modified to offer good compression but they can't even come close to the cooling ability of the Puch or Fred heads.
Both the Dax Gen IV and Neil's Gen V engines have the improved 40mm cranks to give you 70cc ... really about 69.5 or so but close enough. They both are very well balanced so they will run smoother than most others as well and come with better bearings already installed.