I'm right there with many others, I enjoy working on my RC's. On the other hand I can certainly see how this might be a useful service, maybe to someone just getting started.
Tough thing would be to figure out a price, to little and you don't make anything to much and it doesn't make sense for someone to use the service. I'm thinking some newbies would like this but those same people are probably running low end engines which can easily be purchased for $100 sometimes less online, so I'm thinking maybe $30-40.
Might even be able to offer rebuild services but again the hardest part is going to be price. I see people now asking about rebuilding traxxas engines for example, by the time you replace the piston, sleeve and bearings for literately a few bucks more just go buy a brand new traxxas engine and be done with it.
Keep us posted if decide to go forward with this!