Kewl Go Kart barn find what's it worth

I don't think the briggs came on it but the install was professional,started life as a 5 horse but after they add all the roller bearings and stuff they do I have no clue but learning as I go,I had some homemade ones in the past too and also had a kewl twin engine one when I was younger that mom bought me but the most I ever had on it was a single 3 1/2 horse on it but it was fast and handled like it was on a rail and I was a small kid so I could catch the faster carts on the long haul
That is one cool cart. I built some home made carts when I was a kid but never had the money to do a real nice one. How many horse is that engine rated at? I see in the specs it says it's a McCullough MC30 but it looks like a Briggs. Did McCullough make them special for carts?