I'll date myself here ... Having Raced R/C boat for well over 20 years starting in the mid 1980's, Nitro was King and we saw many hull classes based off the gambit of engine displacements and sub types. .21 / .40 side exhaust / .45 / .67 / .90 cu in.
The little .21's had raced K&B's early on being inexpensive, then onto Rossi or Nova Rossi, to O.S. Blue heads Then the PICCO P-5 series starting with silver cased and then the newer Black case P-5's, and some CMB's in rear exhaust and the Red & Green head models too.
As MARINE and water cooled ... 60 -65% Nitro fuel was the go too !! We spooled them up 28 to 31,000 rpm ( P-5's ) and needed to run McCoy 9 "Cold" plugs. At the time we could get custom TI connecting rods or run RPM rods.
That was some great years of nitro racing ... it by in large got replaced with large 21-25cc Gas Class engines and then the FE invasion