Like anything else, it depends on how much is already there and how good you are at hacking. Got a loco with no cab view at all and the mesh has no attachment for a cab, it can still be done by using PEV attachment maker to add an a.cabfront attachment. With that, or if the mesh already has a designated a.cabfront, it's fairly simple to add the interior tag - what's complicated is getting the viewpoint to match the actual model so you don't get weird stuff like the back end of the car you couple onto with the front coupler inside the cab, or a cabview that's 50 feet in the air. That usually involves cloning the cab and adding position numbers to adjust it if it's way off.
Engine specs are easier, Gumbytrains Alco RS3 spec worked fine until some idiot with the DLS cleanup project uploaded the KUID2 replacement with all the data for CAB mode removed, so it only works in DCC mode. To fix that I cloned the original and used content creator to replace the original engine spec with mine, now all my Alco RS3 and RSD5 locos run in CAB mode.
