The other thing to check is the bronze or brass push rod location devise (sorry don't know it's official name)
Basically it is the bit that push rod locates to, it is in the top of the block, you can see it when you look down the the push rod hole. (easier if you remove the push rod first.)
When my belt broke, I broke two of these and it gave the problem of a valve clearance. One of them broke up after a few miles. They are easy to replace, just undo the bolts on the side of the block.
good luck