You will need the engine model to begin with. Just stating the boat type is of no use as many different engines were fitted. Easiest is to take a starter to your local auto electricians and they will match it up. They may also repair it for a fraction of the cost of a new starter.
if the engines are 4 cylininder then the engines are probably aq21d,s. if so these are the same as a 1980 ford granada diesel or a peugeot 504 2.1 diesel.
as stated above, take it to an auto-electrition and have it looked at first as most of the time they can be repaired.
Thanks, I did take it to a guy and so far have waited 5 weeks for him to source the part that confused:my starter motor needs, will check the numbers thanks for the information Angie
As a rule, don't take anything to a boat repairer except as a very final resort. I suspect your 'Guy' was one of those as an auto electrician should be able to source the parts immediately.