Hello @chalcosoma,
Apologise I didn't get around to replying last night.
You can get away with running some pretty aggressive tyres on the stock rims, my friend once went very savage on his DTR (to the point spokes to breaking left right and center ).
Sadly, I am not that clued up on tyres and availability in the States in general, but I certainly would have thought this wouldn't be a hard problem to solve.
It would be interesting to hear what other people have run in the past as I know quite a few members had TDRs.