After having 2 inboards I would recommend not cutting corners.
Your best converting to a chevy power and carb to keep it simple. Raw water coole will be simple and cheap and should last a while with only riser and manifold changes every 5 years.
Here is a good site to get some estimated costs.
http://www.marineexhaustmanifolds.com/
or
http://bpi.ebasicpower.com/index.php
You could find a rebuilt or used 318 but you may end up in the same boat if you run into issues again. Spending more $$ and time at the dock vs out on the water.
Depending on the HP you had now you may be able to go down to the 4.3 v6 Chevy.