You'd need to check with the Ringmaster Roundup organizers to see what's open to the S-1, what's open to other Ringmaster variants, and whether an electric Ringmaster will be accepted.
The S-1 is the only Matt-Kania designed Ringmaster that's eligible for Old Time, and it's the one to build if you want to be a purist. It's an easy build from plans, and if you don't want to build from plans you can check with Pat King in the Vendor's Corner to see if he has any kits. (Other people may sell kits, too - I know that Pat Johnson in Idaho has a modern Ringmaster variant, but I don't know if he sells an S-1 kit).
The stuff from an Electric Clown ought to work -- I just looked and a Clown is a bit smaller than a Ringmaster, but not by much. If Brodak was generous in sizing the stuff it should work.
The Ringmaster's a fun plane. I should build another one.