Thank you for the question XXXXXXXXXX,
Your unit (From the factory) is a Mercruiser R Drive and uses a impeller pump referred to as a high pressure, low volume pump....Very common and is available after market....We use a lot of the Sierra Parts version of this pump...
47 - 89984T 4 Impeller
46 - 96148T 1 Upper housing
46 - 57234T 1 Water pump repair kit / No Upper Housing
47 - 89984T 5 Water pump impeller kit with Housing...
Try this link, the last time I did this it did come out right....I will try the link myself and get right back if it doesn't work.....
http://www.dougrussell.com/partscatalog/components/index.cfm?fuseaction=getcomp&colid=04&coldesc=R%2FMR%2FALPHA%20ONE%20%28Sterndrive%29%201983%2D1990&index=2203
It Worked !!!!
Click on "GEAR HOUSING (DRIVESHAFT) - "R" " and this is the lower unit and parts list....
Marine Mechanic
21 Years Experience as Mechanic, 18 Yrs Boat Yard Owner/Mechanic, Factory Certified