OK, I realize that this posting is a month old but I hope that you didn't buy it. First, the price is very close to new cost and nobody should have to pay that much for a used engine. I had an SS and it was a pile of junk. I found out that the issue on mine later on was that the bottom-end con-rod bushing was missing from the factory! Mine was made just before K&B was sold to MECOA.
Can it work on a mono? That's a good question. My guess is that you have a Dumas Deep-Vee 20 by your description. Outboards generally don't do well on mono's, kind of the opposite of what you'd think seeing how there are so many full-sized outboard boats on the water. The center of gravity is a real pain in the rear as everything has to be moved foward to get it right.

OK, the updated part. I went in and looked at the engine again, it doesn't have the original stock Gold carb but the Red one. The Red carb is for .45's, not a .21. They're a hassle to set on a .21 as the fuel and air can't atomize correctly.