Welcome to the forum Crowley! I admire your gumption! I'm a longtime rider myself, but have yet to take a coast-to-coast trip.
I think the best setup for you would be a Q-matic kit with either a Huasheng 49cc or a Honda GXH50 and a 58-62T wheel sprocket on a Manic Mechanic custom sprocket adapter for your wheel. Either engine will be capable of making that trip as long as you don't rev em over 7500RPM for extended periods. The Q-matic transmission itself is well-suited to extended running. The MM adapter and sprocket for your exact wheel will make chain alignment easy. You may be able to make the trip on a single chain!
If you have any more questions, you're in the right place! Lots of knowledgeable MaBers here!