A home repair is probably not practical. The reflex mirror in an older film SLR is fragile and difficult to remove and reinstall without causing more damage or affecting alignment.

Based on the answers, the most realistic replacement path would be a donor Olympus OM-1 body with a good mirror, then transplanting that part. But even that is delicate work and may not be worth the effort unless you’re very attached to this specific camera.

If the camera still functions and the viewfinder remains usable, the simplest option is to keep using it as-is until another problem develops. Otherwise, have a repair shop evaluate it, or replace the body entirely. With older film cameras, anything beyond a very minor repair is often hard to justify compared with finding another working body.