1981 380 SEL Problems

I seems to be having a little trouble troublshooting this problem. The engine has a slight tapping the tapping under valve cover. I am thinking it is just a rocker needing tightening. My main problem is the transmission. It started to slip in first gear and now it only goes in reverse. Before that I was able to get it going by just manual shifting through the gears to get it to go. Well, my question is would I be able to just replace that gear the is messed up or will I have to replace the whole transmission. There is nothing wrong with the other gears. Thanks.