ok, here are some pics of the 'accident'...
the first screw, that broke, was the one, which holds the connector of the electrical grounding (mass) of the boiler. it's the one on top of the boiler - if you look closely to the first picture, you can see the stump of the screw still sticking in the winding below.
then i tried to loosen the screws, which hold the boiler unto the brew head. two of them were very tight, but loosened after many attempts with different techniques. the 3rd one broke - just sprung apart - you can see it on the top right side of the 2nd pic - still sticking in the winding of the brew head. the 4th one couldn't be loosened at all - no way! so i flexed it off (bottom left). the reason they stuck so heavily is, that they were baked with rust into the material. obviously the gasket ring was so damaged, that, by pressure, water drained ever so little between the two parts, so the screws baked with rust...
then, as i tried to separate the parts (again, baked like cement to each other, even without the holding screws), the rim ot the 4th screw opening of the boiler broke away. as it is cast aluminium, i wasn't too surprised... that would even be fixable, but to drill out the broken screws and cut new windings etc. seems like a bit too much of an investigation of time... so i thought, if i could find any intact spare parts, it would save me the day... ;-)
the learning would be, after all, to dismantle the thing from time to time. this would help not letting it get to that point, but the pre-owners obviously didn't care... :-(
p.s. the use of this type of boiler in a gaggia classic (of which i own two myself) doesn't seems like a good idea / practical in my opinion, as the wiring is really VERY different and the mod would be a heavy one, if possible at all!