a .18 or .21 is gonna be overkill, they would probably rip the guts right out of it..
the 4-tec comes with the trx 2.5r and it has a different shaft than other motors, and the car is probably designed to fit that motor, same thing with the tmaxx..
so I'm not sure if or how well another motor is going to fit.. someone else will have to comment on that..
if thats not a problem I would go with a os .12tz..
you hardly ever hear anyone talk about them but the hpi nitro star .12r xs is a great little motor for the money..