I think the sierra part number for the seal kit is 18-2682. Marine engine.com list that kit for 1958-1967 10hp, but I am pretty sure that the lower units are the same between the two years. Someone correct me if I am wrong though.
That’s a tough one. That was the last year when 10HP motors used the 5 bolt gearcase. From 1958 onward they used the lighter duty 7.5hp style gearcase.
Laings Outboards lists one and I think it might be something they assemble in-house.