$10 to $15 for exhaust cover gaskets, give or take, then a head gasket and an t.stat cover gasket if you get into those. Thing is, the motor was apparently pretty messed up, and it makes sense to clean out those water passages. The wear plate under the impeller usually goes in only one way -- should polish it up, along with the housing. Try back flushing through that nipple -- won't get at all the passages but can pick up some. Let's see, if you want to take a cautious approach, I would do the flushing from top and bottom, replace impeller and clean everything up in the l.u. Then I would consider pulling off the t.stat cover (gasket in hand), and replacing the t.stat. When in there, you will be able to do some effective back flushing again catching much of the water passages. Then put it together and see what you have. (Again, if salt water there is going to be more corrosion.)
edit. with racerone, get to the boat test and see what you have (Big believer in just running them) -- but assuming you have it pulled apart some (at least the l.u.), do some flushing along with the l.u. cleanup then have some fun with it in the lake.
