I assume the fuel pump on the seapump is a carter from merc. Do they make rebuild kits or is my option to just purchase a standard bbc carter pump and transfer the lower assembly onto the merc upper? Dont know the history of the pumps & had some fuel in the seapump oil. Just going to be fueling a 420 ish bbc so i believe stock is fine.

Any advice is appreciated.

Thank you

Fuel in the oil is normal, it's something you should change on an annual basis. (The oil)
pretty sure you can get whatever you need if yours is worn as far as parts go. Also some have drilled an tapped for a bottom plug th make changing the oil quick and easy.