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An engine goes rrrrrrrrrrrrrr. Unless it goes rrrrrrrrrrrr it is a chunk of aluminum often confused with an engine.
Try to SELL an engine off a boat on which it can be demonstrated and You will be reminded of the principle of rrrrrrrrrrrr.
This might not help in this deal, you might need to walk away.

The other principle is that of what it might be worth to him as opposed to what you will have to spend on it to make it work. Because unless it works, it is worthless to you. AND
You can put a thousand or four into it and unless it runs across the lake, it will be worth about what you paid for it or less!
Good Luck.