Transforming a weed wacker engine for a boat ??
Old Guy:
No reason Why You couldn't use almost the whole weed eater. You'll need the blower and fan for cooling. Rewind starter for obvious reasons. Get one with centrifugal clutch so it will idle in neutral. No reason why you couldn't modify the driveshaft for prop shaft. The 5 in height might be a problem. Stihl makes a very good compact unit.