I'd be very surprised if this is at all even close to what you've in mind, on the other hand it's exactly why I built it, for long distance riding with a heavy trailer laden down with camping gear, noms & toys... er tools.
build thread: http://motorbicycling.com/showthread.php?t=24210
1) Engine: 49cc four stroke Lifan (Honda CRF50) clone
2) Transmission/Gear box: 3sp internal autoclutch
3) Tires/Rims/Spokes: Alexrims DX32 double wall alloy
4) Hubs/Brakes: Quanta 20mm sealed bearing, Avid BB5 disc
5) Frame: fabricated
6) Exhaust: Piranha 50
7) Trailer or everything strapped to the frame? both
8) How would you do your riding (how long would you ride)? 100+ miles per leg
Granted mine is a hybrid, but the Lifan/CRF50 (about $250) is a high quality clone with an actual, integral transmission (3 or four speed, auto, autoclutched or standard - your choice) and is a beautiful little engine, the full lighting & charging with electric start is a handy bonus & it'd be fine by itself stock. There is however a vast selection of aftermarket preformance parts ofc.
Figured I'd throw it out there lol