1. Yeah, only goes upto 68tooth- 68 vs 70 won't make any real difference really, maybe one tooth up on the pinion but tire ballooning and ambient temp will make more of a difference to running temps and such.
2. The difference is so small it won't make a difference- different brands use different pressure angles on their pinions so that is more likely to cause premature wear if anything; just set the mesh correctly ( tiny bit of play but no more ) and it'll be fine. Any mod0.8 or 32p steel pinion with a 5mm hole will be fine - Mikes are great, if not the RobinsonRacing ones are great too.
3. I got someone to do that for me when I upgraded, but it looks simple enough based on a few guides:
http://www.rc-monster.com/forum/show...0&postcount=51
Seems like to keep it 2 speed, you'd need to buy the topshaft from the new emaxx/erevo tranny, remove the single gear, then drill 2 holes in the correct spots to install your original topshaft gears- just a matter of sliding them on and wiggling them about until the slipper is in the correct position and the gears line up correctly too, then mark & drill the holes to pin them in place.