Replaced Cylinder Head seemed to lower CC

[oneir0naut0]

I recently laid down my 150cc scooter. I took it to a mechanic and he said I had cracked a cylinder head. He replaced the cylinder head. Afterwards, my 150cc now behaves exactly like a 50cc scooter- barely wants to push 30mph up a hill and maxes out at 45mph on a flat stretch.

I'm very naive when it comes to these things. Can replacing a cylinder head actually lower the cc of an engine? At first I thought I was being paranoid, but the scooter is now for all intents and purposes a 50cc scooter.

If yes, would the mechanic be obligated to fix the situation? Is it just something else going on with the scooter?

Thanks so much in advance for any help on this.