I've been thinking about this off and on over the past few days and I think a knife hinge, like you would see on a toy box, available at most hardware stores, would work. I guess the trick would be to get the right size and position it correctly. I'm going to putz around a little and see if it will work.
Okay. It's a couple hours later and I am editing this post to report that it worked. I dug out a knife hinge that I think you could buy at any hardware store for maybe $1.29, drilled two 1/8" holes, tapped 6-32 threads in the holes and used 2 6-32 machine head screws and a few washers to mount the hinge to the easel in just about the same spot the gas cylinder once was. Works just fine.
Even if you had to buy the 1/8" drill bit and the smaller ones that lead up to it - 1/32, 3/32, maybe 7/64, you would still be under $10.00. Wait. You still need a tap for the threads - you might be another 5 or 6 bones out of pocket if you had to buy a tap and holder - might as well get a cheap tap and die set for $20.00 for the next time you need some threads in something.