im new to this stuff too and hopefully this summer i'll be able to afford the expenses for all this bike stuff
. As far as I know a frame that has alot of spacing inbetween the top and bottom tube works best as the engine would be able to fit inside the frame. But you might be able to mount on your top tube and seat tube since its seem to have so much spacing from the seat anyway! I have a mountain bike that sort of like the frame you have but even worst because the top tube is in the middle of the seat tube, hence giving it not space for a "normal mount". Technically, any engine can be mounted, but for the special "rack-mount" type engines it'll cost like around $100-$200 more just for the mounting kit. For that much you could just get a new bike, but yours seems "possible". I also know that kits more friendly w/ 36 spokes wheels for the sprocket mount but that is about it. You'd need someone with experience to reply though, as i never mounted a engine, i can't wait to though. I have a Schwinn voyager that I'm ready to rebuild and mount a engine on.