Klinger sounds familiar. They made the boiler glass gauge of my olympia (which holds up to 300bar, just in case...)
I didnt know that they also make gaskets.
What i'd recommend is silicone. Most silicones are safe for use in food processing machines, because its super inert, just like the teflon coating in your frying pan.
Teflon is also an option, but its way stiffer than a sheet of silicone. If you have a badly uneven meeting surface, go for silicone, since it will squeeze into the scratches.
If you have the machine apart, you could grind the surfaces perfectly flat and smooth.
Silicone should be easy to obtain, just make sure its foodsafe and temperature proof, get a stencil from your boiler and a exacto knife and start cutting. For screw holes you'd best use a hole puncher. Same goes for teflon. Most gaskets i use are about a millimeter thick. If you use thicker or thinner material than the old gasket, be sure that your boiler screws are not too long (boiler wont be tight if they reach the end of the receptive hole) or too short (not enough thread engaged, danger of messing up the thread).
Don't know about fiber or cork gaskets.... i use those for engines. You can get them at the garage you trust working on your car. Whole other situation there, those are oil proof and most likely NOT foodsafe, but a good option to stop your on/off switch from wiggling, or similar. But i bet there are also foodsafe fiber gaskets.
If you are not so fond of dodgy hand-carved gaskets, make an EXACT drawing of your stencil, buy some material and a case of craft beer and drop all of it off at the next architecture bureau or another place where they have most likely a laser plotter. They can cut you a perfect shape.
Silicone and fiber is easy for lasercutting, teflon i dont know, but DO NOT cut PVC or unknown material on Laserplotters, nor have it cut by other people, the fumes are causing cancer.
Another thing i found that might help:Conti Prestina Espresso Machine Restoration 101 (Completed and Indexed)