I've used Carbon Copy Cloner to copy the internal disk to the 120, and am running off the 120 as I type, booted over Firewire. Everything seems to be present and working properly.
The next step is the hard one: moving the disk inside the iMac. I have no doubt it'll fit, physically, or that it'll boot and run. What's worrying me a little is the iMac's design. I'm going to have to pretty much completely disassemble the base to get the drive out, and reassembling it will require some thermal grease. Now, I haven't had to mess with that stuff for years, but in this case, it's imperative to ensure that the heat pipe that carries heat away from the G4 has a good thermal path to the heat sink on top of the case. Whee.
This is a slightly unusual feeling for me. Normally, hardware holds no mystery for me. I've got all of the necessary tools, and know how to use them. I don't have any heat sink grease that I know of (and I don't think Paul does, either), and I'm not sure, if Radio Shack carries such, that it's the good stuff. A trip to Mankato might be called for to get some of the good stuff. The last thing I want to happen is to have this machine crater on me. That makes me just a little nervous.