Just discovering this great thread! For years I've thought that a 1986-On Honda XR200 chassis, and an engine like the RT180 Yamaha engine would be good ingredients. RT180 Desirable aspects: -Inexpensive, reliable, an old tough design. -Cooling system self contained, can file or grind down fins for clearance. -Could use or ditch the oil injection system. -Narrow, relatively light weight. -Big flywheel, made for grunt of a trail bike. Xr200 desirable aspects: Brakes are drum (narrow, cheap, reliable, light). Steel tubular chassis, easily mod'd by the home tinkerer. -Seat, tank, and fender mounts easily mod'd for your wants (adjust rake, shorten, etc). - 18 & 21 wheels, big clearance for rear tire. Big square shape engine bay to accept engines: Had I not found the following 03 Beta 250 Rev3 recently for a good deal $1,500.00 (30 days ago), I would want to build such a xr200-rt180 bike: Honestly having a newer used (15 years old is new to me!) hasn't made me ride any better. Just being honest.