Unfortunately, obsolescence is a thing. I know that's why QSI went to the Titan design instead of continuing the Q1a / Revolution decoder. I don't know if components of that era are still available. Fact is, most technology is obsolete after 10 years. Unless the component requires some kind of programming, you might try an electronics surplus store.
What caused the malfunction in the first place? e.g. short in the motor, weak field magnets (increases current draw), etc.
Personally, if you like the locomotive, I feel it's worth the investment in the Titan - U. There are so many improvements in the Titan I cannot list them here. There is just so much more operation and sound realism.