You can fit any engine in the TT nitro that you can physically get in the frame.

If you get a replacement with a slide carb rather than a rotary one,you'll need to rearrange the linkages somehow,using a rotary carb would be a lot less trouble.
Also be aware that engines with a pull start fitted will need the plastic pull start removing or the engine won't go in the frame,on engines such as a Rossi and possibly the Picco engine,removing the plastic pull start assembly exposes the loose pull starting shaft that runs in the internal one way bearing,so unless you can make a new backplate and starter shaft you won't be able to use the engine.I made a backplate conversion for the Rossi engine in the TT nitro we race.

Your best bet is probably one of the OS range of pull start engines,removing the plastic pull start exposes the starting shaft,but it will not come out of the engine,so all you need is the one way bearing fitting on the end of your starting wand.

Allan