Re: what did you do to your motorized bicycle today?
Another alternative to consider would be a rack mount but on the front end to drive the front wheel, that would make for an easy install but will need to do some welding on the front fork to make the engine mount. You could use a rack mount kit or just do a custom job with a cg engine to keep costs down and there will be plenty of replacement parts available.
just need to sit the engine far enough forward so the drive sprocket is directly over the wheel, then you could either use a rag joint or a manic type adapter up front. A tank could be made to mount on the handlebars or a tube could be welded to go over the engine from either the front or rear mount tube to keep the fuel above the engine. A pulse pump could also be used if the fuel tank needs to be mounted somewhere else on the trike...