Two words: Eric Gorr
Not trying to brown nose, but Eric built us a 150 which is actually a 144cc bike. It's a 99 KTM125, but we also have some other mods such as FMF pipe, FMF silencer a rad valve etc...
The bike pulls much stronger everywhere. I believe Eric did some epoxy work to the cylinder for us to give it more mid as Michelle was racing it in the womens nationals (amateur). The bike is pretty authoritive, if not a little flat up top but it pulls hard, never bogs and even pulls my 200+ lbs frame around no problem so you know it's making some torque.
We just installed as a test one of the new iCAT ignition gadgets that just screws inline with your coil wire - basically what we found is that the bike now revs extremely quick and picks up lower in the powerband. Interesting gadget as it totally changed the personality of the KTM powerband.
Next stop I believe is to send it back to Eric and have him port it for more upper mid and top end - I believe he told me he does a pretty close job to what the new KTM125s are running, and what with the big bore kit, it should run fabulous.
So to answer your question - to get your bike to run more like a new Katoom, send it to Eric and he'll do basically similar porting to the 2001-02 models and you get it at a fraction of the price of a new bike.
Suspension - if yours is anything like the SX good luck. I love the KTM, it has been bullet proof reliable, but we went down like 4 sizes on the rear spring and up one on the front.
Robbie Horton in 99 was going down like 4-5 spring rates and going way up on the front forks (all the Moto Planet guys were). Even recently the KTM team has been doing this to get the bikes to level out.
I can jump on the 99 with my size and ride it with the stock rear spring on it no problem - although I just can't take huge launches etc...
Dunno who these guys recommend here on the site - I've tried both PA (pro action) and Factory Connection and at the time neither seemed that awesome to me. Good luck.
MP