There isn't a formula for converting HP to CC, I'm sorry to say. There's a lot more to horsepower than just the engine size.

For example, a well tuned up 250 cc engine will have more horsepower then an old one. There are too many other factors involved to convert directly.

This model is Troy-bilt's heavy duty machine; if that helps. I checked for other equipment using the Briggs' 250CC engine; some of them are rated at around 8.5 HP.
A good way to compare performance is to check the size of the branch the unit can process. A unit that can handle 6" branches is more powerful than one that handles 2" branches.

How the debris is processed can be important, too. Some systems use knives and flails; some have flywheels with blades imbedded in them; others use combinations of tools to turn wood into chips.

This unit has 2 chromium steel chipper knives and 12 replaceable cast steel flails; and handles branches up to 3" in diameter.