This tire-drive motor is only 40cc:
Gas Motorized Bicycle 2 Cycle 40cc Tire Roller Drive Rear Mount Bicycle Motor Kit - Live Fast Motors
Do you really need 80cc? The idea behind these bikes is that you supply some of the power. The engine is really just an assist. Is that such a bad thing? Any vehicle that is intended to pull you plus its own weight is (or should be) an actual motorcycle, with much greater HP, structural strength, and weight than a motorized bicycle.
I don't know about the reliability of this motor, though. If you want bullet-proof reliability, get something like a Stihl chainsaw motor and adapt it yourself.
http://www.stihlusa.com/WebContent/C...ison-Chart.pdf