I just bought an old, rusty mountain bike at a garage sale that I want to fix up to take to and from the Metro. It's a Diamondback Sorrento from sometime in the '90's. The bike rides, but the rear gears aren't shifting. The gears are Shimano Acera X. I want to replace the rear derailleur with a low end replacement. What do I need to know about compatibility? Will any Shimano rear derailleur work? Other brands? Thanks in advance.