Regarding dirt track tires, check out Duro. I picked up a 3.50X18 and a 4.00X18 for one of the kid's street tracker project bikes that have a tread pattern almost exactly the same as the Carlisle and the Pirelli MT53 - and as the old dirt track guy whose shop I bought them from said - they even have the lightning strike (the zig zag) through the tread blocks. Don't know how long they're going to last though, as they feel pretty gummy. DOT approved.
Before you order, look at the supplier catalogue to make sure you order the correct ones as Duro has a number of different tread designs.
Pat Cowan,
Vintage Motorcycle Fiberglass
by Pacomotorstuff