While Bob's suggestion has merit, any empty rifle case is likely to be heavier than a plastic 12g so I suggest the issue of weak ejectors will remain or perhaps be worse once the planned conversion is done. Of course some owners of double rifles prefer non-ejectors but if you are going to have ejectors they are better to be working properly to throw cases clear or eject into the hand as many reloaders even of shotguns will do.

I would definitely agree with Bob that the ejector mechanism built into the for-end is often neglected when the gun is being cleaned and maintained. Usually there are only a couple of screws to undo and the whole mechanism can be lifted from the fore-end to be cleaned and oiled. The fore-end is a nice trap for moisture, dirt and grit and the mechanism contained within invariably gets gummed up pretty badly in guns that maybe nicely cleaned on the surface but no deeper. Bob's advice is good re stripping and cleaning.