If I remember right Minimac started with an engine displacement limit of .50cid for 2 strokes; not sure about 4 strokes. My GP Extra .40 doesn't even fit into that category because I installed a ST75 in it after the ST45 failed to perform.

I really like the idea you bring up: 80" max for monos, 65" for bipes (60 just seems too small for me to consider IMAA-legal), with an engine limit of maybe 62cc? This will encourage guys with venerable Midwest Extras, H9 Caps, or DP Ultimates to compete with inexpensive powerplants such as up to the G-62. Keep the nonscale rule for Basic class and either keep or lose the Unknowns. I like the Unknowns but they might keep some people away. Sounds like it would be a lot of fun!