Great news! Disassembling baked in and rusted parts is one of the hardest parts of a restoration. Thanks to Paul Pratt and ira for suggestions on getting that element loose.

Magnus, if you're saying the group is still glued on, what about applying a propane torch to burn it away, taking care with any flammable solvents you have on it?

Yes indeed thanks to Paul and Ira, i would still be soaking that element if you guys hadn't given some input!

You may be right Gary. Even if i can remove most of the gasket the group is still rusted on. I will try to look for a propane torch when i have some time on of these days. Anything specific i should be looking for when it comes to power or temp?

Propane is cooler than other torches but hot enough to burn off a gasket.

Depends on the gasket material. Considering the age of the machine I'd do any gasket burning outside & wear a respirator or at least make sure that the wind is blowing the smoke away from your face.

Since the only thing on the lid now is the group it makes it a bit more easy to handle. Instead of buying a burner and respiratory mask or have to do set up outside couldn't i just soak the gasket in water or a mild descaler for a considerable amount of time? The gasket isn't rubber or plastic so wouldn't water soak it up and soften it given enough time?
This is partly because it wouldn't be as time consuming but also because i wouldn't have to get a lot of tools i might not need again for a long time. If a burner is necessary i will get one, but if a few weeks of soaking can do the same i would definitely prefer that.

A really aggressive liquid paint stripper would probably soften up the gasket so you could scrape it off. Coat it liberally & cover it up so the stripper can't dry out. Might discolor your lid but bead blasting or polishing would bring it back.

Also the kind of propane torch I'm describing isn't expensive. It's a small propane bottle with a torch nozzle for the end. Using it so you don't inhale the smoke shouldn't be too hard. It's your choice, though. I would get frustrated not getting the group off the boiler plate for many more days to come.

I have been able to remove the group! The gasket was stuck pretty good but after som initial hard hits from a flat screwdriver the gasket loosened and could be pressed out between the group and lid. It seems like a lot of the gasket wasn't really in too bad condition but was stuck around the group studs. After getting all the gasket pieces out i discovered that the group itself was loose and a bit of "wiggling" meant i was able to get it off completely. The surface of the gasket area is a bit scratched but i doubt it will be a big problem after some cleaning and polishing.
3 of the 4 group studs could be remove without damaging the threading but the 4th one is not budging. I have left it to soak in some penetrating oil and i hope it will do the job.

This means that i'm now done with the boiler part of the disassembly. Next part will be the steam and hot water tap as well as the group. With the Prestina thread that Gary have written i hope disassembling the steam and hot water tap won't be too difficult. At least things probably won't be as stuck as they were with the boiler.
I've been looking at local stores for a spanner wrench but i haven't been able to find any at all, they seem to be quite rare around here and are mostly available from online auto tool vendors.

I am a bit short on time these days so there probably won't be any major updates for at least a couple of days. Thanks for all the input guys, it's really makes this project seem like it's possible. A year ago the idea of spending this kind of money on a espresso machine and then actually have to restore it myself seemed like a pretty crazy far-fetched idea.

PS: Pictures will come when i can get my phone to work properly.