frames
From what I've been able to tell, diamond type frame mountain bikes and early std bike frames work the best. The older mountain bike frames have the right tube diameters and angles to allow simple bolt-up of the engine to the frame (not to mention they are stronger frames) likewise, the earlier std bike frames are set up such that they allow easier frame bolt up without having to use much for adapters etc. to mount. The newer large diameter tube frames tend to be a little more of a pain to work with and require some fabrication skills. Avoid bikes with twist grip shifters as the twist grip for the shifter occupies the same area as the twist grip throttle for the engine.