I did an MGT conversion. I didn't want to over complicate things, so I used all stock parts plus the forward only kit. I just opened up the tranny and pinned the oneway, to the second gear actuator (is that what it's called). Mine is first gear only because I wanted something to get crazy with. I got the smallest spur gear and have a 16 or 18T pinion with a 7XL. This thing is stupid. My son actually drives it up tree trunks and does back flips with it. He's also flopped it flat on it back and it bounces back onto its feet. I can't say enough about this truck as a play toy. If you want more speed, just go with the second gear and gear appropriately. Just make sure that you do set the slipper. If you get no slip, it will break the second gear shaft inside the tranny. We've done it half a dozen times. Good thing the shafts are cheap and easy to change. I used 1/4" angle to make a motor mount. The whole conversion took me about 5 hours to do.
BTW, there are only 6 friction pins in the slipper, using only half the available holes. Get another set and use all 12 holes.