Capt. K, you will see many times used boats for sale ads that read, new powerhead zero hours. So many times. This is the same thinking with old cars, get it fixed and sell it before something else go wrong.
If you spend any more than you have to to fix it, each dollar is going into the pocket of the new owner. Newer motors will not get you more money to speak of. Working motors of any vintage is all that matters on a 20 yr old boat. If you spend 7K on a newer pair, vs. 1.5K for a rebuilt lower unit, the sell price will hardly $$ matter to anyone. And there is no warranty - that would have a value.
I decided to replace the engine on a 150K mile old Nissan Z, paid $4K for rebuild, car was only worth about the same at the time, bad move one would say, but I intended to keep the car and nothing else was wrong including original auto trans. I drove the car with the motor & same trans to 1/4 million miles. Bad investment? Just the opposite - the best money I ever spent since I drove 100K more for only $4K. Only because I drove the value out of it. If I sold the car, all of it down the hole.
You sound like you want to sell it, obvious not firm in keeping it. Until you have the plan firm, can't really make a financial decision. I presume money doesn't grow on the trees in your backyard, mine neither.
