I picked up a new TRX 2.5 R and Revo pipe off of ebay real cheap (~$70 with engine, pipe, header [sold for $10 on ebay], coupler, shipping). I slapped the 2.5R and the Revo pipe onto my rusty (had to make a new mounting wire for the pipe) and it is incredible. A lot of people ***** about the trx engines being hard to tune and unreliable. I personally have found the opposite to be true. All of the problems I've had with TRX engines have been related to issues like the poor fuel tank on the t-maxx and fuel system obstructions. When all else is well, a properly tuned TRX 2.5R is one of the smoothest running most reliable engines you can put on a rusty without overpowering it too much. Actually the trx 2.5r is way too much power for a rusty unless you are nice and easy on the throttle, so I would be very reluctant to go to anything more powerful. If I were looking to change from a trx 2.5 or 2.5 R to anything else, I would look into the .12 racing engines.