Re: four stroke engine.
I've run the Honda 50 and Titan 50...both excellent. I've heard the HS 50 is also very good. Although not 3 hp, they will easily climb hills with gearing. The Titan into a Nu Vinci will pull me and a fully loaded (75# or so) trailer up a 20 degree rise at 20 mph at the top. I've also run the Lifan 97cc single speed with a belt drive that wasn't bad on hills. Very long hills would get it with the trailer, but it was acceptable. With even a two speed it would have been great. I don't own one, but the HF Greyhound seems to have similar performance.
My favorite for an inframe is the 50cc Lifan with the auto clutch. Lock it in 2nd or 3rd for automatic, or use the gears for performance if you like. They are rated at 2.7 hp, so plenty power too. Mine is in a chopper style frame, so I don't tow with it, but hills are no problem, and 40+ mph is available if you play with the gearing. To use these you will have to do some frame mods to mount them properly though.
Looking back at it the engines have been less of a problem than the bikes for me. My only problem was drowning the engines when riding in a heavy rain for extended trips, and this was when the fenders kind of channeled the spray onto the engines. Rack mounts didn't even suffer from that.