You can put a .21 in with no issues.
All you need is the following;
Engine mounts from the CT4-S
Retaining screw and nut for the clutch assembly (.21 engine has a smooth shaft unlike the NT.16)
U bend exhaust header to rotate the pipe back towards the front. Unless of course you want it sticking out the back and ready to be ripped off on each 'flipping', which is what will be happening quite often with a .21+
You will need to adjust the throttle setup so that it will worth with a slider carb.
You will also need to relocate the battery/receiver box. I simply lay mine on it's side so that the servo arm would not clash with it now that the throttle geometry was different due to the slider carby. It ends up looking very 'factory' and very neat.

It's a pretty easy conversion, I dont know if I would even call it a conversion, more like a 'swap out'.
If you need part numbers etc, give me a yell.

Rgds
Anthony