[willy]

If that Sea Drive will run, run it till it dies. Guys here that have had them say they performed well for them for years. The problem with the Sea Drives is parts availability and the fact that a lot of mechanics won't work on them. So depending on your expertise you could repair it and run it for years. There are parts available but you will have to net surf to find them. The alternatives that some of us like is the installation of a bracket on the back after glassing in the sea drive hole and installing a outboard of your choosing, with a 150 considered by some as minimum, and unless your redneck from the Carolina area 200 generally considered max.
I would work on that sea drive and see what the compression is and go from there, it could just be a fouled up carb problem from poor maintenance or not being closed up right and sitting for too long, or possibly a ignition component gone bad. If the compression is good that motor could be tuned up and run for years good luck