Since it sounds as if the motor is not original, It's hard to tell, but if it says 420hp, then they may be merc 420s (454, 420hp). I have never had to provide a engine serial number for a seapump kit. I think you just need a bravo sea pump kit. I get mine at napa auto parts or the local marine dealer.
Someone will correct me I'm sure, but i believe that the merc part numbers are as follows:
Sea Pump Kit: 46-807151A14