oskuk: Shouldn't be all that difficult. However, if we were talking about a 505er with missing motor, you'd need to make sure that you get the SM112-1, not the SM112, as the motor needs to sport the matching top plate design for the chassis plate. Whereas for an exemplar with a dead SM100-1 you could grab either version, as you could still swap the wrong top plate of the SM112 for the correct one of the broken SM100-1.
Greetings from Munich!
Manfred / lini