Originally I had planned on just having a F/A-18 for show, without the engine or the electronics. Here in lies the problem.

How hard/easy would it be to install the engine, whether it be a turbine or ducted fan, and all of the electronics, after the jet has been assembled?

Not sure if that came out really clear, so think of it this way. Say you took a kit from Yellow Aircraft, which is just the frame of the jet. You build all of that up, without the engine or electronics. How hard would it be to install them afterwards? Is it even possible/feasible?