As to the speed of the H12c, my late uncle built up a C from around90% new old stock parts but with a used Franklin 6V335. I partly overhauled the engine and gas flowed the heads and fitted a later longer duration new cam for an injected motor. We never had the power output measured but at a guess it must have been around 260hp at an indicated 3300rpm and it topped out at 86mph (GPS).
Andy.