Hi jacksmith,  I have a 1993 DT200wr and it has been supermoto'd (160 rear & 120 front).  I've had a go on an original one (with knobblies) and mine is better .  I run a 14 tooth front sprocket and have kept the rear standard.  I do reckon that you should run a 150 rear (otherwise the chain takes gert lumps out of the tyre!)  Brilliant bike mines good for about 90mph (tucked in). 
 
Have fun.