For a quick check on the condition of the cam and the valve seats, remove the valve cover and the rocker arm shaft. Place a straightedge across the ends of the valve stems. They should all line up pretty much evenly. If one of them protrudes (nearly always an exhaust) that seat is receding, due to use of unleaded gas. If it hasn't gone too far, a machine shop can install hardened inserts.

To check the condition of the cam, turn the engine by hand and measure how far each push rod moves up and down. Once again, they should all be identical, perhaps with a minor variation between intake and exhaust (though all four of each should be the same) If one or more of them comes up noticeably short, the cam is worn out, and must be replaced, along with the lifters.

If everything seems to be OK, reinstall the rocker shaft, adjust the valves, clean the inside of the cover, put on a new gasket, and put things back together. The valve seats will go for a while as they are---using a lead substitute additive is probably a good idea---but will eventually need to be replaced with inserts.

It might also be worth removing the timing cover and checking the condition of the fiber gear.