The specs for the OS .91 show:

SPECS: Displacement: 14.95cc (0.912 cu in)
RPM range: 2,000 - 16,000
Bore: 27.7mm (1.090")
Stroke: 24.8mm (0.976")
Compression Ratio: 10.1 : 1
Weight: 550g (19.42oz) 24.3oz w/muffler
Horsepower: 2.8 BHP at 15,000 RPM

This is a theoretical horsepower IF that engine reached that RPM. Attempts to reach that RPM with that engine will end up with a pile of aluminum and steel dust.

CGr.