A couple of things:
-Custom cars work fine
-Tracks (the few I've tried) don't. Admittedly they're much larger than the default tracks, so that might account for it.
-Though people say the source isn't public, I've found it at http://revoltsource.awardspace.com/
if anyone wants to port back the updated version to the Xbox (was originally Xbox code given to the community, they took that and made a custom PC version)