Is this a matter of milling a few thousands of the front protrusion of the cylinder until the desired overall length is accomodated by the frame opening? I have a NF .22 dual cylinder model that is in need of it's missing mag cylinder.
Generally,a .22 Magnum cylinder made FOR a NF should have the same o.a. length So that shouldn't be a problem.
Where the "fitting" might come up is the cylinder bushing face,but again,this isn't usually,or a major problem,
Where the problem CAN be is the ratchet. It might allow your hand or pawl to rotate it. I have seen them drop right in,and others that need to be "timed" in the ratchet for smooth rotation(and in rare cases,to rotate at ALL).
This fitting of the ratchet,obviously is NOT a job for the "inexperienced!
I had a .22 Peacemaker,in .22 lr. that I picked up an almost new .22 Mag cylinder for in a gun shop. It dropped right in,and "all" I had to do was lightly stone the front bushing a little and the ratchet to get it silky smooth and in correct time.
Hope you are lucky and the one you find "pops" right in with NO fitting,or that you have a qualified gunsmith close to you to do the work.