You don't even need a new upper casting. It is pretty easy to remove the fixed crosspin, cut a slot and fit the adjustable cam plate and screw. I have done this to two older boats with no problems.

To cut the slot simply drill a hole where each end should be, drill as many similar holes in between those as you can and file out the remaining metal to form the slot. Borrow a later casting to use as a guide if you can.

To complete the job, you might want to drill and tap the lower casting for the small rake adjusting screw, but I'm not sure how important that is. Matt Bounds might have an opinion.