Do I need to remove the OEM exhaust before installing this?

Yes. The factory exhaust must come off completely. This system bolts to the existing catalytic converter flange, so you'll need basic wrenches and jack stands to unbolt the old system and hang the new one in its place.

Will this system fit if my catalytic converter has been modified or replaced?

As long as the catalytic converter flange matches the OEM bolt pattern, this catback will mount correctly. If you've upgraded your cat or downpipe, verify the bolt spacing before ordering.

Does this come with a muffler, or is it just piping?

The exhaust assembly is a complete catback system—piping, muffler, and single tip all included. You're getting one complete unit, not individual components.