I'll also be the guy who just says, that if you don't mind the risk, you can just use it as-is. I wouldn't use it on a popular lake without registration but a smaller lake in the middle of nowhere....I doubt that anyone will bother you.
As for the bill of sale, you can certainly find a form online if you want (
https://eforms.com/bill-of-sale/mi/michigan-boat-bill-of-sale/). You can also just find standard language online as well. Usually when I or someone else does it, I just write out that John Doe is selling to John Smith this 1978 Beretta with a 19xx motor for the price of $0. If there is a trailer include that info too. Put the hull ID and any serial numbers that you can find on the boat, motor, and trailer. Put the 1998 registration number in there as well. Write both parties addresses and phone numbers, then have both parties sign it.
I'll mildly disagree with GA_Boater in the sense that you can write out the bill of sale for the widow and have her sign it. My guess is that she may not know what to have in there. You could write one up and she can feel free to do her own version if she prefers. I find most sellers don't care so I bring my own pre-written with blanks for all of the boat/motor/trailer info to fill in at the time of sale.
The Michigan Secretary Of State (SOS) link that briangcc posted is what you should be reading.