Try the OS 12tz. I have one in my MTX-3 and it is a pip. Here is a quote from a post I made on another forum about my experience with this motor at the 5th annual Paris Memorial Race:

First, Jlock can tell anyone that the OS 12tz motor is no joke. Just box stock and broken in the right way, it can hang with the best of the fast motors and at a lower temperature to boot. It has a smooth power band and good top end. If geared properly, hanging with the Mugen X-12s, Novas, Mega Piccos, and JP Mods is not totally out of the question. Jlock, after a 40 minute run, pulled his off the track at a cool 196 degrees!! Jlock's Mugen X-12 motor got that type of power produced by the OS but nowhere near that kind of temperature range (usully around 258 degrees).

It is the easiest to tune of all of the motors that I have in my possession (I have Nova's, Sirio's, Rossi's, and Picco's in my motor arsenal). And, the cost of the motor and the rebuild beats the others hands down.