Thermal,
I'm campaigning an '01 125 SX in the GNCCs and local Harescrambles. It's a good bike with the mods that Marcus mentioned. Stock it's too tall for medium to short guys, too pipey and the suspension is way to stiff. Gas milage is insufficient for anything but play riding. For starters, get the oversized SX gas tank (~2.9 gallons @ $235). It doesn't change the ergos a bit. Then get the suspension set up for the woods. These two things are an absolute necessity. If you are used to a 125 in the woods then the rest of the stuff isn't as critical. I found that a Moto Tassinari reed cage and Pro - Circuit pipe/silencer setup to work wonders for the motor. Mo power everywhere while also making it smoother with a longer top end pull. I never went with a flywheel weight but I'm considering one now. I'm 5'8", so I cut the subframe to lower the tail of the seat about 5/8"-3/4". This doesn't sound like much, but it is amaizingly noticable. Especially in the tight technical sections. Overall, the bike is an excellent choice for offroad use as it's incredibly reliable and easy to maintain. Much more so than even the modern Japanese bikes. Personally, my only gripes are that the power will never be as broad as my 200 was and the seat is brutal. (Nothing seams to help the seat situation except standing up.) If you are concerned about the power of the 200, don't be. Believe it or not, the 125 makes the same or more power than the 200! The 200 just makes more power in the lower rpms. I rode my '00 200 MXC like it was a 125. Plus, if you get the exc, the wider ratio tranny and heavier flywheel will smooth things out even more. After the last GNCC, I'm again trying to decide which bike to buy for next year.....125 SX (with all the new suspension, oversize bars, smoother layout, better turning) or another 200 MXC (perfect woods motor). You should try to find someone in your area to test a 200. You may be surprised!! (PM me if you want more specific info.) Good luck!