The cam, the thing screwed onto the back of the cylinder rotates when key is turned and triggers a mechanism just below the hole you would screw the mortise cylinder into. Assuming this is the exact cam you have on your cylinder, then you have an adams rite lock body. If you put the cylinder in and it works the lock, then the lock body is working.
So to answer your question if you wanna DIY, get the cylinder first and install. If it works, great. If it doesnt work, then you know the lock body isnt working like it should and you should call a locksmith.