I've seen them with all sorts, BMW, Bukh, Nanni, Beta, Yanmar, Volvo
Yanmar seems to be the most popular, it was an OE spec engine so it is only natural people look to that for a replacement too.
The only one of the above list that didn't really fit to my mind was the Bukh 20, which required moving the P bracket as well as building an extension to the engine compartment. It also resulted in the boat sitting stern down in the water.
The 500/501 hull was originally designed for a small petrol engine, a bit like the Dolphin but called an Atomic Gas Engine.