There are many different impellers today.-----Not the same 40 years ago.----Time to measure some shafts and housings .----Find a $20 one that will fit can be modified to work.
Do you push in the key and hold it in while cranking it over ?-----Yes , no or no idea how the primer valve works?----Filter is inside the primer valve.----Remove 4 screws to find it?
Pushing the key in and holding it in while cranking it over ?----Does electric primer valve click when key is pushed in ?----Checked filter on electric primer valve?
At one time folks did not like the idea of a big outboard hanging on the back of a boat.-----It was deemed cheap and unreliable.-----The L-drive was a way of hiding the outboard.-----How times have changed !
Pressure test and see exactly where the leak is.---Then repair that leak with say ----" lumiweld" ---brazing rod.----Better yet is to replace that housing.
Obviously it had water in it and froze.----Any pictures of the inside.-----I would say that housing is warped to a degree from welding.-----Or mating surfaces are damaged , resulting in the leak.----Easy fix for me here.
All depends on your budget I guess.----Cut out that damaged part.-------Make an exact piece to fit the cutout.-----Keep that part in place with a plate inside the boat with 3" overlap all around and some rivets.