The ALT has 2 wires (not counting the black ground)
Red/Purple which comes from the 50 amp breaker (constant power)
Purple which comes from the ignition switch (switched power)
Setting timing even on the water under load by ear can be catastrophic. Motor set to run its best with timing to far advanced which can put it where preignition starts. Most likely there is other issues besides timing
Cheap and reliable is bore it and rebuild what you have. Less cost and no need to reprogram the ECM. Boring will give a very slight power increase but that's about it. Trying to build power using your 305 would be better to leave it as-is and go boating
I would be worried if the hull was solid. Most hulls that old could need some major hull work. The comment of " no gas and believes it runs" would mean it hasn't moved in a long time
Sure hope its better then I'm thinking
