Idunno. Might take a bit of trial and error.

Changing from a (Baseline) Rossi 4 plug on a Rossi 61 engine to the hotter 4-c plug will tend to have the same effect on midrange/transition (overly simplified here) as dropping the nitro content in the fuel.

I always ran my Rossi 60 and 61 engines on standard 15% nitro fuel. I have not experiemented with one in quite a while, and I do not recall trying FAI or 5% fuel at any time.