Well, if you don't know the weight of the aircraft (fixed or rotary), surely knowledge of relative engine power output is of little or no use anyway !
The standard test for piston-FW power is usually a performance climb, but parametric methods are more definitive. John Lowry's book
Performance of Light Aircraft is a good baseline for parametric performance theory.
The approach taken there is generally however to look at the overall degredation in excess power - so poor compression, and dead flies on the leading edge, all bunch together.
G