Pirelli MT66 is what I was thinking. I’ve seen them on Pats bikes at the bike show. They come in a 80/90-21 which is only a 1/8” wider than your beach bums and probably smaller on the OD. There is also Shinko 230. You’ll still have to run tubes if you have spoke wheels but they won’t leave the rim if they pop like bicycle tires will
Yeah Norm been planning a build for my new design 2-speed with a Tilly block maybe the 263cc although small block builders I know say the smaller 236cc can make more power because it is a stronger block capable of higher compression. When I have the money to start I’ll make a build thread then. Before last year I would have built it and rode it out there, but now I must have a cage around me on public roads. Been wanting to go to that event for years now.