You've got a couple options on that bike. I just browsed ebay and came up with a newly-listed replacement carb for $ 75.00 buy it now. Just search "Bear Tracker 250 Carb", and it should show up. (Sorry I can't post a direct link....Im typing this from my phone).
Since the Bear Trackers use a spigot-mount carburetor, you can replace your existing one with a same-sized (34mm) carb off another machine. Keihin or Mikuni would work. HOWEVER, the replacement carb needs to be a CV type, and you will run into tuning and throttle cable fitment issues if you switch to a Keihin or other model of Mikuni.
If I were you, Id find another good used Bear Tracker carb out there and swap it onto your bike. Easiest and least hassle.
If your existing carburetor is in otherwise-good shape, it is often possible to drill out the stripped main jet if you have a steady hand and havent already damaged the threads in the carburetor. Ive done it before and will probably have to do it again. Just work slowly and try a larger easy out again when most of the brass is drilled away. You may not be able to get it out, but you've already damaged the jet itself, so why not give it a shot? You may surprise yourself!
