In my experience, the only thing that comes with a new motor is a glow plug and instructions - since most cars use different flywheels and clutches you need to buy a separate set up.

I have 4 cars and 6 motors - they all have motor mounts, flywheels, gears and pipes already installed to just drop a motor in the chassis and hook up the throttle and fuel lines and go play.

I wouldn't transfer back and forth - it weakens screws and is a hassle