I'd have that whole cylinder removed and sent off to be
checked/rebuilt by a good qualified repair station. Our 172 is in the
shop right now due to one cylinder intermittantly running cold and not
making power. We had the jug pulled and upon first examination, the
intake valve had lead deposits on the face/seat and was only sealing
sometimes, yielding zero compression during one check, and 74/80 the
next time around. My A&P suspects the guide and or stem may have some
slop too, so we're just going to have the valves in that one jug
rebuilt since the other 3 are fine and the engine is only mid-time.