Steahly in Oregon makes flywheel weights for KTM's. I have a 12 ouncer on my 98 200 mxc. They sell new for around $100. I'd sell mine for $75. (I have a 520 MXC and I'm planning on stripping the 200 and selling the bike.) Steahly has weights in 8 and 12 ounce sizes. With my hot-rodded 200 it helped it hook-up in the mid-range without making it sluggish on bottom.
Steahly is the one. But for the cost of only $100, I would stay away from used because the weights are fine-threaded to mate to the center of the stock flywheel, and those threads are soft. If the '98 200 has spent anytime underwater, the flywheel housing most likely has had moisture trapped inside. That means the inner threads are probably corroded (and pitted) which will make the easy "spin-on installation" a real mess. Having fresh threads on a new weight, IMO is worth the few extra $$$ than getting a used one with chewed-up threads
The flywheel has been on my 200 less than 2 hours. I bought it last fall, tried it once and didn't particularly like it ( I like a quick, barky response). I put it back on so a friend's youngster could try the bike to see if he likes the weighted 200 better than an xr400. The flywheel is still as new.