I have heard a few people complain about the stock mufflers being very restrictive on KTM and Husky enduro bike. You might want to get the matching FMF Turbine Core II w/spark arrestor. It should make the bike crisper, but I don't think it will help with the fouling plugs.
My Husky CR125 when I got it was spoogging terrible and fouling plugs like mad. I got it used and it has a Pro-Ciruit pipe and 304 shorty silencer. I tried differenct oil ratios from 32:1 to 50:1 but finally settled on 40:1 The shop manuall says use 32:1 for break-in and no less than 50:1 after that. I dropped the needle a clip position and switched from blended synthetic to full synthetic and went to one plug hotter (from B9ES to B8ES) It stopped spoogging but still fouled plugs a lot. I then went one main jet leaner and it stopped fouling. It still didn't seem right though, bogging down low, so I went one size leaner on the pilot jet and readjusted the air screw and now the bikes runs great. Almost like a differnet bike. I'm not getting any pingging so I don't think I'm too lean.
What is funny is that my son's Yamaha 85 still spooges a little after re jetting and adjusting, but it doesn't seem to be fouling plugs.