What Alex said. The Nexstar is covered in Monokote, so you can repair it with Monokote with no problems.

As far as the engine. The FXi is sold ONLY on the Nexstar. In fact, you can't even buy one on the market. You can only get it with the Nestar plane. While it's got a few cosmetic differences it's pretty much the same as the AX. I say that it's closer to the AX than the FX because of the remote needle valve mount. On the true FX engines the remote needle valve mount is molded in to the engine backplate. If you crash the plane you can pretty much count on that bracket snapping off, and you have to buy a new backplate (~$15) to fix it. On the FXi and the AX the needle valve mounts on a metal bracket that is bolted on to the back of the engine. In a crash this will simply bend and can be straightened very easily. The FXi is a great running engine and will do fine on any 40 sized plane.

Ken