Yep, Arwyn is totally on the money. It's got to come out! That is a pretty impressive damage pattern which looks like both sides have already been used by the wear patterns. Re machining and sleeving will be necessary after rebuilding the lost alloy. Billy Bellinger might be worth a call for repair or replacement or a gearbox specialist like Ashcroft. The stainless plate repair on one as damaged as yours would be a bodge job and I don't think it would be a suitable solution however I have often thought that stainless inserts fitted to a good bell housing would protect the opening when wear occurs allowing the moving parts to contact the housing.
OOP's see you are in the states, Try a TR club etc
Difficultly in having the slot welded back to reclaim it is that the bellhousing is made from Elektron which is a high magnesium alloy. Unfortunately, welders prepared to weld Elektron are few and far between. Not sure bell housings are available new as my Dad bought the last one from the factory circa 1986 for our 1968 Plus 4!
