A .21 can be done, but it would be challenging. The rc10gt already has drop in conversions for one. Also Buzz the carb can be reversed so it can face the correct way, and a slide carb is really easy to set up in an evader just switch the servo around, and make a longer brake linkage. I did mine in 10 minutes. Also if you want crazy power the os .18 tz has more power than some big blocks, and would be fairly easy to fit all you need is a new exhaust since it is a rear exhaust.