You could buy the prevalent 2 stroke chinese in-frame kits. They are cheap, however these require some tweaking to set up correctly. This is the kind of kit I have. Most of the kits that are available will have their advantages and disadvantages. There are a few threads on which kit is best. http://motorbicycling.com/f4/kit-best-1764.html
The two stroke kits are the best choice, but don't automatically rule out four stroke and electric. I've got two Schwinn Varsity and two KHS street bikes that are just asking to be motored. I'm debating which power to use, maybe one of each...
im powerking all the way its been the best kit to me 10K in travel and its cheap to get but thats for the 48cc if you go 70cc drop in a new gear like a 38t or 36t after break in and put new motor mount bolts trust me always on any kit instal new motor mount studs get grade 5or up m6 type bolts the best there is will never brake crashed 3 times on my last bike and never snaped a bolt even reused them on a difreent build and kept on going
I've got 4000+ miles of cross Town/Country on my 80cc(69.7) BoyGoFast and going. Have gotten Quality parts from Live fast and Dax and Kings motorbikes the Boy GoFast has been tougher than nails and is now cheaper priced than ever. The Cast flashing could have beeen cleaner on of the ports on mine though so if you dont plan on doing it Id go with any of the last 3. Gooood kit other than that. actually I dont see but one option on Ebay that has the older one I got. Runs great still. I mean great. almost amazes me with its generous Torque! Fter I ported it I took it to a Motorcycle shop to a guy who races dual engine pavement 2 strokers and asked if he could do a better job of porting he gave it the throttle response test (my Jet is boared out to79 or 80 ha ha, passed with a bit of suprise the off the line test for torque amazed him so much I think I hurt his feelings. I don't know if he thought I was playing with him or what but he kinda put his guard up after that. Id like to gao partners with him in the off season. No hard working DUI Getter wants to be walking in miserable rain when he could ride on of these I proved it possible in my area. So ?