I had that very same year DT250 in the Philippines 20 + years ago. IIRC, I just pulled the oil bottle, the pump and added a line from the tank straight to the carb. Then fiddled with throttle linkage for a year before I ran across one that worked off a Husky. A fellow on thumpertalk just ordered one from here:
http://stores.ebay.com/Terry-Motorcycle ... 34.c0.m322 No feedback if it worked out for his old yammy. They come from Thailand. IMHO, the throttle cable is the only hassle and with pre-mix, you'll get the proper mixture all the time. I silver soldered a copper tube to the top of the carb for the husky to work. 1/4 turn throttle and good to go!