Is it en euro cylinder?
If yes, you should measure the length from the screw-hole in the middle.
like here you need to measure lengh a and b. It's usually given in mm-s and the increment is 5 mm-s. Example: A 30/40 lock of this maker and that type.
So do you need an exact same model (Cisa with yale 8 para?) as a replacement?
Any euro profile lock can be changed to a thumbturn with a thumbturn kit, but not sure how accessible cisa tumbturn kits are there.
The inside thumbturn works fine, but the outside key just goes round without operating their locks.
I sounds like the cam is broken. (By cam I mean the grey thing at the middle inside the lock. Sometimes it's built in with the tumbturn. Here is a non-tumbtunrn version, they usually look very similar.
See if you can jsut get a replacement cam. But dis and reassembly can be a bit of a hassle if you are not experienced with euro cylinders. I'm sure you will manage, but just so you are careful
