Hello, my name is XXXXX XXXXX I will try help you with this.
I'm really surprised that after a timing belt break that only one cylinder was affected.
What you need to do next is carry out a cylinder compression test to make sure all the compressions are the same.
I think you will find a few more cylinders have been affected by the timing belt failure.
I tested compression before removing the head. Three cylinders OK although there were marks on most pistons. All valves were removed and checked before re-fitting.
The issue I am now concerned with is the extra effort required to turn the camshaft when it's trying to operate the new inlet valves. The new exhaust valves are OK.
It might not be from trying to operate the new valves. It could be due to some of the other valves being damaged. You already aknowledge marks on the pistons. That has to been made by valve contact. It could be the valve guides have cracked or slightly bent valve stems.