valve shims?

I had a problem with the 98 missing on 2 cylinders and after playing with it I couldn't get it started. After reading and posting here in the forum I found out it was the camshaft magnet, I replaced it and it did start but it still misses I talked to a local technician and he tells me the valves need to be shimed. Is this true, has anybody done this and what parts am I looking for?