IMHO it would be well worth the time and money to install the engine.
If I were the buyer, this is how I would think:
1. I need to tow the boat home.
2. I then need to install the engine.
3. Since the engine is not in the boat, I have no real way to test it.
4. See #3, the uninstalled engine is potentially not useable.
5. See #4, I am going to spend a lot of time finding this out.
6. See #5, I would think who has a non-working boat and a good uninstalled engine? If it actually worked, it would be installed and the boat would be working.
Kind of like the ever-popular Craigslist ad "Car for sale, AC not working, easy fix". If it was easy,
YOU would have done it