How does one Kitbash?

I was thinking about Kitbashing leeferr’s C&O Kanawha into a Berkshire or a Greenbrier or a Hudson and Ben Neal’s USRA 2-8-8-2 into an N&W Articulated. The thing is, I don’t know how to kitbash stuff. I don’t want to wreck one of my K&L Locomotives by kitbashing a clone of it (since I heard that was possible). I watched a couple of tutorials on kitbashing, but they were all related to K&L Trainz locomotives. Any help is greatly appreciated. I use TS2009, btw if that helps anyone.
You simply clone the locomotive first. Cloning means you make a copy of it, so the original remains intact.

Using Content Manager, you create then open the clone for editing in Explorer (Right-click and choose open for edit, I think in TS2009, and Windows Explorer opens up).

Edit the Config.txt file and change the Username to the new name so you know that this version is your own, otherwise it's difficult to tell which version you're placing down on a route.

After you've made the change to the username, you can then save and close the config.txt file.

Now, using, Gimp, or even more expensive programs such as Photoshop, you can then edit and modify the textures for the locomotive you want to weather.

When you are satisfied with your changes, you then save the .TGA files. When you save, ensure to uncheck RLE compression, otherwise the asset will not work properly.

Close your Explorer Window and commit the asset in Content Manager.
Hi. I'm trying to kitbash a copy of a copy of a K&L C&O Kanawaha to resemble a Pere Marquette N-2. To do so, I need to swap out the pilot for that of a K&L PM Berkshire, and move the headlight to the center of the smoke box door. However I know nothing about programming and do not know how to kitbash like this. I'm open to suggestions on how to pull this off.
You cannot change the model itself unless you have the Blender or Max files. All you can do with someone else's creation is reskin the model to conform with your idea for either a fictional RR of your making or another RR you have pictures for. Make sure to read the license to see if it's okay to upload before doing so. You can reskin to your heart's content for personal use.