Cool, have you popped it off yet? check the diameter printed inside the clamp & the model number normally stamped on the back of the steel chain guide. I maybe able to help as I have a tonne of this vintage.
Ive done nothing its late now I was thinking if I should just replace it with another part but can you buy a gear mech which pulls up from the bottom ive not messed with my bike for ages just rode it and kept it clean lol
There are tonnes of bottom pull mechs available as long as you have the right clamp size you'll be okay, you can pick up a cheapy here or on ebay for not much.