In Canada, the Rebuilding Centre is located in Ontario, and rebuilds all Mercedes parts for Canada. They do engines, tranmissions, alternators, axle assemblies, torque converters, starter motors, water pumps, and cylinder heads. These are the ones I know of, but they might do more.
There is a core charge. This is the ONLY "factory rebuilt" parts stream in Canada. The parts usually carry a three year mileage limited warranty (engines and transmissions) or a one year warranty (other bits).
A friend of mine bought a cylinder head for his M103 and it was about the same price as having one refurbished locally. Much better value. And his warranty was intact even though he installed it himself. The one caveat was that the warranty would NOT cover anything determined to be an installation related problem.
1998 C230 "Black Betty"