I haven’t found the lid yet, I have uncovered the hole front of the tank and I don’t see a lid, I thought it should be near drain entrance (inlet bafel)
The whole top comes off. Just finish uncovering it and pull it right up.. Its not that heavy but you will start with some kind of pry bar. That's an old tank. The exit pipe is probably clogged as well. You will kind of need a whole new system. Try infiltration chambers for ease of installation.