[bluedog]

To the valve clearence check and the needed gaskets:

Don't confuse cylinder head gasket and valve cover seal.

When checking/adjusting valve clearance, you do not remove the cylinder head, and therefore you don't need a cylinder head gasket. You don't even touch that.

You only remove the valve cover and if you need to replace shims, you take out the camshaft(s). Cylinder head still stays in place then.

People who must know that recommend, however, to replace the valve cover sealing with a new one. Otherwise, the Engine might spill oil trough the reused one.

But that brings me to an other good Idea, for finding the reason of the power loss.

Do a Compression test. That can show you, if all valves are undamaged (sealing off the combustion chamber properly) and if the cylinder head gasket is ok.

Complete loss of Compression on one Cylinder would indicate, there is a valve burnt up, or a leak in the cylinder head gasket. As long as all three cylinders have more or less the same pressure value, the valves and the Cylinder head gasket are ok, and you just have to take care of the valve clearence itself.