Yep, looking at your station stops you have no Trainz versions registered - you need to have at least one version registered to get your unique "KUID" number in content manager, otherwise you can't create your own content in later versions. Once that's done (I'm gonna go with the config.txt since that's what you mentioned, altho it's easier in Content Creator);
1. Open the loco for edit in explorer.
2. In the Explorer window open the config.txt file in notepad.
3. Find the Interior tag, make a note of the existing KUID number, change the number to the KUID for the new cabview.
4. Scroll down to the KUID table and change the same number you just changed in step 3. Alternately you can use the replace utility in notepad to replace all if you know how to do that.
5. Save and exit, back to content manager, edit, commit.
Note you're editing the loco to change the interior KUID, not the engine spec - unlike MSTS where every engine has a .eng file which contains the physics, Trainz engines reference a separate engine spec file, to change the physics you would change the engine spec to reference a different one in the config.txt for that loco. Unless you're one of them DCC types, in which case the physics don't matter.