I had some success refurbing an Isolette a few months ago.
My dad had used it last in the early 70's. By the time he gave it to me around Christmas, it was completely frozen. I didn't want to spend a $100 either, and had much fun learning how to disassemble and clean this camera.
The frozen focus ring was solved in much the manner described above. It wasn't too tough. I removed the whole lenses assembly from the camera, then gave it an overnight soak, then wrapped the front and back of the lens assembly in hockey tape. Very gentle application of twisting from opposing locking pliers, and it unscrewed. Once apart, the whole thing can be soaked again. Every glass element was fogged (probably by the decaying AGFA grease) and had to be cleaned front and back, and every thread had to be scrupulously soaked and cleaned. Before reassembly, I gave the threads a wipe with silicone lubricant. It is now silky smooth.
The shutter and self timer on it was also frozen. Further disassembly and soaking got much of the gunk out. It's amazing how much solid flotsam will come out of such a little device.
The trick on reassembly is to VERY lightly oil the few spots which might need it. I found that the bare unit with no lube would freeze, so I experimented with adding fine camera/machine oil from the head of a pin. Too much, and it would stick again. So, I would soak it out in naptha, dry it, and try again with even less!
The fast speeds >1/10 run smooth and in perfect time. The self timer works like a charm. Slow speeds are wonky -- occasionally perfect, but usually slow and sometimes stuck.
In reading through Certo6's site (where he kindly provides a lot of details) the biggest thing I lacked was the ultrasonic cleaner. I think that would have done the trick to remove whatever is junking up my slow speeds. As cheap as the ultrasonic cleaner seems to be (at Harbor Freight) I couldn't justify having yet more crap hanging around the house after a single use.
I got lucky on the bellows for my Jsolette - they seem to be the leather kind, and certainly don't have any light leaks.
The exterior of the camera came up really nicely with Silvo taking the tarnish off the aluminum top and bottom. A little bit of Armor All wiped on the leatherette made it look like a 20 year old camera instead of a 63 year old camera!
The lens is easily collimated, simply tape a piece of hampshire frost or wax paper in the film chamber. Then you can collimate the lens while looking right at the film plane.
It's a great user camera. Handheld, I never want to go under 1/10 second anyway. I have an accessory rangefinder (unfortunately my Jsolette doesn't have a shoe mount, so I have to just hold the finder.) Using the rangefinder is much recommended -- I am spoiled by the deep DOF of 35mm and digital and have been off on my "estimated" distances.
Does anyone know a good solution for filters? I read somewhere that they take a Series V push-on filter. I tried a surplus "Series #5" that I ordered from Camera Depot, but it won't slip onto the lens. I would love to have a yellow and red filter handy for this little marvel.