Yamaha had a 60 that was rotory valve called a JT1? me and almost every one my age learned to ride on one, so should be alot around. Suzuki and kawi also made some rotary valve small bikes, don't remember much about them or if they made one just for dirt. I would recommend a yamaha GT 80, its a cool little bike, 2 stroke but not a rotary valve, should be easy to find and get parts for. If a 4stroke will work, you cant beat a honda XR 80 or even a 75, I would agree they are "bomb proof". When I was a kid used a XR 75 for my paper route, it kinda got handed down with the route. At least four paper boys thrased that poor bike over about 5 years on a 15 mile route. I think it just turned to dust one day.