After trying lots of ideas, I got my Faemina Baby to work. My original seal was intact, but just was shrunken from its original diameter. My solution was to leave the seal on but add an o-ring to the recess at the top of the original seal. To get the right size, I went to my local hardware store and they let me try all their o-rings to see which one entered the cylinder with enough pressure. One required a huge amount of force for my arm to press the piston into the cylinder, but another one required only a moderate amount of force. I went with the moderate one and it worked and the Baby's arms made it go into the cylinder effortlessly, but with no leaks. I possibly could have gone with the tighter o-ring because the leverage with the arms is much greater than my hand pushing it in. (I just took the piston and cylinder to the store, not the whole machine, so my test just involved pushing the piston with o-ring into the cylinder by hand.)
I took photos of the o-ring package info, the o-ring on the machine, and the first espresso that I made. I didn't even bother with a fine enough grind or heating the portafilter. I just wanted to see if I could avoid leakage.
I originally posted a photo of the o-ring package here. But I now realize that the store gave me the wrong o-ring package. They opened up several for me and put the fitting o-ring into the wrong package. I know this because my package had a cross-section (height) of 1/16". I think the one I have is really 3/32", using a tape measure. Even though it works with no leaks, I would like to see if I can find something even tighter.
I apologize to people who saw the 1/16" size and found that it leaked and did not solve the problem. When I find out the maximum size that fits the cylinder, I'll report back and I hope I don't make any mistakes.
To the best of my knowledge, the o-ring in the above photo actually is this:
2-3/16 in. OD x 2 in. ID x 3/32 in. wall