Hmmm.....
IMO, I'd go for the Kawasaki KE100 or KE125. Rotary valve single that was produced essentially unchanged (in the KE100) all the way through the early 2000's. And the 125 was essentially that but 25cc bigger.
Yamaha singles were pretty good, too, esp. in the later 70s with the advent of monoshock, although I can't remember, offhand, if the small cc bikes HAD monoshock.
If you're looking for something off the beaten path, go with a Hodaka! Huge cult following and all the parts / support you could want out in the Internet world.
But, unless your really set on a 2-stroke, I'd personally go for a Honda SL/XL100 or SL/XL125! But, they're 4-stroke singles. Everything is still available, and with minimal maintenance (frequent oil changes), they should last forever!
On the other hand, for a REALLY versatile bike (or set of choices) go up to the 175cc class - there you'll have great bikes from all four Japanese makers - Yamaha DT175, Suzuki 175/185, Kawasaki KE175, and Honda XL175. All very versatile yet still lightweight for 'round town and light trail work.
Good luck,
Kirk