How many horsepower is a lawn mower engine which rated at 173cc ?

It's impossible to say. You're showing the "displacement" of the engine (the volume that the pistons displace as they move). The displacement is not exactly related to horsepower in any simple way. While engines with larger displacements often have higher horsepower, it's not always the case. The most significant factors are compression, displacement, and engine design (which obviously includes compression and displacement).