Outboard lower unit interchange

Boy I don't know without part numbers or casting numbers, and I don't have any of my old books immediately available (fire cleanup) and some are just plain missing in action. But they are gasketed with a thick "rope" neoprene seal, and I would suspect chineseoization rather than matching. I did look up one or two of the online exploded parts views, and the gearcase IS sold as a "pair."

https://www.crowleymarine.com/evinrude/oem-parts/outboard/1968/18802a/gearcase-group
You also might try "the right stuff" or other recommended sealer. Do you know where it's leaking? You can device a pressure/ vacuum tester by using a fitting in the drain/ filler plug. Then you can do a bubble test just like a tire. Don't forget the gasket for the little screw in the very bottom for the shift fork. It can leak.