[mr0077]

I was racking my brain trying to figure out what I'd do in your situation, and concluded the first thing would be to try to contact the guy that built the car and ask what he did to the motor (and then try to figure out if the motor build in your car is the same one he did). If that is not an option, then waaayyy more info is needed to help you. What idle RPM have you got and is the idle rough (lumpy or racey)? If a 700-800 rpm relatively smooth idle, I'd think it was a mild build and could use 91-93 octane, but if it won't idle below 900 RPM and sounds and acts pretty radical, it may require higher octane fuel. You could check the compression, but interpreting those readings is tricky and depends a lot on cam overlap and compression ratios.
You might try running the tank down, putting in a few gallons of 93 octane and drive it gently, listening for pinging. If you get pinging or rattle on light throttle opening (in gear) you probably need higher octane (but it could be an ignition timing issue), and you could either add racing fuel or Xylene to get the octane up. If no pinging or rattle, continue to use 93 carefully.
If you don't feel comfortable interpreting and testing, take it to a good mechanic in your area and ask them to give you their opinion. Lots of variables involved in this without taking the engine apart to be sure of what you have.
Contacting the previous owner or engine builder would be the best option.

Any other ideas out there? Not an enviable situation but an interesting question...