Uhhhhhhhhhh, none? Like bolt right in? None I would think, though I'm not sure. If you want lap belts probably easier to just install some? My shoebox doesn't have any. Getting thrown into the dashboard is traditional. I'm a purist.
So, do you need a seat and seatbelts? It's not clear from your post. Seatbelts are not that hard to install if you already have the seat.
There's actually a 51 vic on eBay right now with the same thing. I wanted to pull my eyes out with a s**** when I saw it.