OPV needs adjustment with a pressure gauge afterwards. The one in the pic is probably beyond salvation.
The big gasket should be replaced, typically every 2 years. It's also very cheap.
Shower screen and group base below it should be removed and cleaned typically every 6 months.
Remove the group base before the outer ring!
You forgot to remove the group base, it's the round iron thingy held by two screws after you remove the shower screen. More in next step.
This is very similar to, if not a gaggia classic. Even a machine at such a state can be 100% repaired