Do a search. There was a guy in Montana or Wyoming who did it; said it required some minor "narrowing"/machining of the individual gear wheels.
I seem to remember someone here from South Africa had a dealer do it.
Another option is obtain a Canadian '98 WR250 (two cycle) gear set; this model was imported there one year. Other than an 18" rear wheel and side stand, it looked identical to the YZ, even all the plastic. It had a few different internal parts, one being hte WR trans (lighting coil and extra FFW; porting?) Perhaps MACE can offer some thoughts as he used to own one (that I wanted, baaaadddd). Many others from up your way bought/brought them over the border.
Team Suzuki (off road) does it by swaping, mixing and matching DRZ/RM250 gears.