I saw a BMW "Funduro" on Craigslist today for $1500, same engine as the new $8500 USD Goldstar...so DIY? Some good tires, a nice exhaust and a bit of fettling and you'd be there. Loads of Bullet & other make tinware available. Of get a Suzuki 5-speed S40 650 single and start with that, or a KLR 650 Kawasaki, or....
The venerable Pre-Unit Bullet 350/500 has a 50 year production history and its parts are still plentiful. Even the newer UCE machine bits are easy to source. Elsewhere here R211 shows how to create a crank mounted ignition, and once you have spark without an ECU then a carb is no trouble at all to segue into. Nothing wrong with a long stroke, hydraulic valved, carbureted pushrod single. Don't forget to replace the OEM primary chain with an upgraded part and de-fang the tensioner.
The new OHC 350 Meteor is virtually the same stroke as the old machines, and the upcoming (450cc?) Big Bore kit from H's will allow it to meet or exceed the pushrod motors performance levels, and do all that with a smooth, counterbalanced engine.
Lots of options here.