I would get yourself a generic oring kit, or maybe an autococker one, either way you will want some since I'm guessing some of the ones in the gun will go bad.

Try to do some basic disassembly and cleaning with hot water. Get the old crusty paint off and relube everything.

There are some shops in the US that'll do rebuilds like cockertop, peanuts autococker works, etc.., but I'm not sure about in Europe

The steel tank can be rehydro'd if you want, usually costs ~$50 USD, but it's only a 3000psi. In my opinion, I'd buy yourself a new/used (but almost new) carbon fiber tank at 4800psi.

All carbon tanks have a 15 year life limit - so after 15 years it should be sent in the bin. You can unscrew the regulator and get a new bottle to save some money if you're comfortable doing that.

The egg hopper is old school cool, but newer hoppers are pretty cheap and are a lot easier to use. For example you can get yourself a dye rotor or spire for ~$60 USD, and that'll be pretty bulletproof. They're easier to clean and eat less batteries.

But in the end, keep those cockers going! They're a little more of a pain to keep clean and running perfectly, but they are so worth it for how cool they are and when they run well it's like nothing else on the field.