Hi Gil,
If it is the older Red Head they were excellent stunt engines. Frank Warburton used a RH .35 in his Tony to win the British NATS. And it is no small model. Like Randy always says, 28% oil with about 20% castor. Sounds like you are on the right track. For stunt use a 10X5, 10X6 Top FLite paddle blade style prop seems to work good.
BIG Bear
RNMM/AMM