I took a close look at the holes needing o-rings or gaskets. Here is what I saw for the heating element part inside the boiler and the water and steam taps outside the boiler.
Here are the element insertion holes from inside the front boiler plate:
Here's the fit of the element inserting into those holes:
Here's the shape of the gasket indentation in the steam and water tap holes from the outside surface of the boiler plate. The photo isn't definitive but eyeballing it shows it's a flat indentation.
All of these look need flat gaskets, not o-rings. So I begin to think that maybe a washer on top of that would help and perhaps some additional gasket material. The parts diagram only shows a 16X2 part number 2396, which is labeled in French and abbreviated too (!) as "JOINT TORI. 16X2". If I interpolate they're saying it's a joint o-ring 16mm i.d. by 2mm thickness. But McDave tells me the stems of these tend to turn. That suggests that maybe the softer rubber o-rings he's used aren't as stiff as the Viton that's needed here. Let's see if the Viton will hold in place better. My .07 Viton sheet is almost 2 mm thick.
I also looked at specs of the material that would be suitable. It says at McMaster-Carr:
"Buna-N (Nitrile) Rubber—Use with water, salt water, waste water, sand, ethylene glycol, hydraulic oil, petroleum oil, gasoline, diesel fuel, and citric acid. Not rated for pressure. Temperature range is -65° to +275° F. Color is black."
Steam temperature at about 10 bar per a steam table is over 355F. This rules out Buna. So, the o-rings I was sent would melt down. Added later: Wait a minute! Espresso machines push water through the group at up to about 10 bar. The pressure in the boiler had better not be much more than 1 bar, maybe 1.2 bar for a commercial machine. At 1.5 bar temperature would be under 233 F and this machine has a 1000W element for a 4 liter tank. And yet something with higher temperature rating than Buna seems a better choice for surrounding the tip of the heating element itself.
"Viton® Fluoroelastomer—Use with water, salt, detergents, varnish, ethanol, and biodiesel. Not rated for pressure. Temperature range is -40° to +400° F. Color is green."
So, Viton will work. I received some .07 inch Viton sheet today. I like the thickness and balance between flexibility and hardness. It isn't squishy like the thicker Durometer I've got. I can use a punch for the 16mm inside diameter of the gasket. The lip on the steam/water tap hole is 2mm across and flat. I would use an 18mm punch first then center a 16mm punch inside. Alternatively it may be better to order a circle cutter for situations like this. I have not found flat Viton gaskets of these measurements online. Phillip Canuck offered and sent two alimentary gaskets. To fit the heating element ends they'll need to be 3mm thick with a 12mm inner diameter. If they don't fit I'll attempt to cut gaskets from my Viton sheet.