Check out the Honda CT110 and CT 90 "Postal Bikes". Last sold in the USA in 1986,they are dead nuts reliable, top speed of about 50mph, haul lots of weight, and sip gas. The knobby tires and frame design are great for trails. Ebay usually has one or two for sale...
The best retro-minibike out there (IMHO) is the Bonham Tote Gote. They have a multi-stage torque convertorhooked to a 5 to 8hpBriggs and Stratton, and were used by Hunters in the 60's to haul deer and gear out and into the woods. They have an adult sized frame, and simply can't be stopped. Top speed is only 25mph or so, but they will haul two hunters at once, and drag a deer at the same time, even up steep hills. I owned one that I restored ten years ago, and never should have sold it. Never gonna be street legal, but a heck of a cheap trail bike. Do Google searches for "Tote Gote", some people misspell saying Goat, so you can find results either way. Their are sites and forums for them, too.
For modern stuff, I think the Yamaha TW200 is hard to beat. (More of a dirt bike than a scooter, for sure) Street legal, 60-65mph, fat tire, and a very long production run. Lots of web support, tech support, parts, ideas out there.
Always been a big big fan of the 50cc Honda Ruckus, too.