I'm sure you'd be aware of this, but
LDraw is a good start for models if they can somehow be converted to whatever format M&B uses. They've replicated thousands of parts, all with the correct proportions and textures and everything. The parts are ultra low-poly, however, so they won't have quite the roundedness of some of LEGO's video games.
As for the legal side of things, LDraw parts are released under the Creative Commons Attribution license (
more info here). This license is very easy to work with, and shouldn't constrain the mod at all.