There is no seal kit for that motor, because it has a different prop shaft seal than newer models. So you can buy an 18hp seal kit plus the correct prop shaft seal, or you can just buy all the parts individually. iboats sells parts.
Or, there is an individual that makes up seal kits from parts. He may see this and contact you.
EDIT: That elusive prop shaft seal that fits your '55 is part number 303495