Pick up a late model 450cc four-stroke dirtbike and pick up a set of extra wheels for it.
If there were a nice SMX track nearby, I think that'd be the shit for rapid learning with minimal risk, w/o hefty investment. You'd think that with all the motard bikes around here, some enterprising individual would get something exclusively SMX going. You could also give ice racing a try during the colder months.
http://www.2wf.com/content/view/31/28/ http://dirtbike.off-road.com/dirtbik....jsp?id=331345