The OS .21 TM is a direct bolt in and will use the accessories that came on the TRX 3.3. The Picco .26 Maxx or JL Red Dot will require a big block motor mount. All three of these engines are great quality and have more power than the stock TRX 3.3.
Suggestion Vector I would run the 3.3 after you get chassis ready for what ever kind of track you are gonna run then if you think you are ready put the ponys to her. Just a suggestion now could save you a bit of $$$.
dude, i had a few revos with lrp .30s and the tranny and drivetrain held up great, and i didnt baby it by anymeans, so if they lasted through that, I'm sure the os 21tm will be fine..