Has anyone ever replaced the middle part (circled in the picture) of an adjustable cylinder with a 12-tooth gear? Is that even possible? Or would you recommend against trying it?
Please don’t. The cylinder is one part for a reason.
This plastic part works a lot because it moves the bolt. Trying to replace it will compromise it. Even if you succeed it will jam the lock soon.
Just use an appropriate cylinder. The U200 supports all cylinders, as long as they have the emergency function (this means: you should be able to put two keys into the cylinder).
If the key is too big for the U200, you may trim the key handle a bit using a metal saw/grinder. Cutting the handle will not break the key.
There are also many alternative cylinders with smaller key handles.
If you don’t want to cut the key, just try looking for a cylinder with a matching key. I bought a cylinder on Amazon, and the key fits. You should be able to measure the key through the clear plastic. So you don’t have to open the box to check if the key fits.
Source of the image: U200 manual