Townboy:

The HB engines are loop scavenged and mildly timed, so they aren't the really high powered engines available with the Schneurle porting system. With some careful port work the power can be greatly increased, but engine life and fuel economy will suffer.

HB 40 - 0.79hp @ 13,000 rpm
HB 40 PDP - 1.18hp @17,000 rpm
HB 61 - 1.43hp @ 13,800 rpm
HB 61 PDP - 1.80hp @ 16,000 rpm

These are from the factory spec sheet.

Bill.