That price for the gearbox is not too bad. If I have no luck fixing the old one I'll go that way.
I don't want to buy a new catcher mower just for the few lawns that I use it on. The gearbox itself looks perfect, chain & sprockets are still good. Have never needed to adjust the clutch plates/corks. Just put a new belt, new wheels & pawls & pinions, so it should go for years once I get this sorted.