Power ?

Where do you want power? Low, mid or high? or all around ;-)

Is your silencer a shorty? If so, it was really designed for low-end. The longer silencers boost top-end. I would suggest advancing your timing a degree or two, it will give you more power on top but it will affect the bottom. V-Force reeds will help - they cost about $150 - and you have to take a hacksaw to your intake, but well worth it in performance - they make more everywhere.
If you decide to have it ported, Tom Morgan of TMR racing does an awesome job on the YZ125. I have one of his motor's and it is unbelievably strong. Cost me around $500 in mods if I remember, though... I hear Eric Gorr has a killer overbore setup, but if you want to stay at 125cc I would talk to Tom. P.S. Motor mods in and of themselves (if done properly) won't hurt reliability. However... the more power an engine is asked to make, the more the reliability tends to head south. A 31HP engine will typically outlast a 38HP engine by a while. Ask it to make more power & spend more time at higher RPM's and the reliability goes south even faster!