If the piston is cracked (which you still dont know for sure) then its size/circumference or some other physical property of the piston has changed. In which case it is probably scrapping up and down the cylinder wall and damaging it.
If left like this then a piece of the piston could break off eventually and hit off the valve and then in turn break that too, destroying your engine.
Thats all worst case senario.
Maybe get someone to do a compression test on the cylinders.