Guessing you are talking about my mod release? Here:
https://www.se7ensins.com/forums/threads/release-forza-horizon-project-underground.859211/
If it's what you are referring to, then yeah you can go ahead and rerelease it updated just give credit.
Convert the texture to tga or xpr and modify it by removing the grey part of the texture that covers on your new imported model. It will remove that section elsewhere on the car but that's pretty much best option you have.
Do you have my xex's along side that media.zip?
Edit: Just read what you wrote previously. Everything loads on the main menu and it hangs on a loading screen forever?
Have you tried a clean fm4 installation? Tried a different profile?
Wouldn't it be just a xbox labelled game on the windows store like the current ones? Sure enough games in the windows apps folder can't be touched otherwise it asks to re-install the game, but there's already been ways to modify the files. There'll be some modding on the win10 versions no doubt.
If you have a good idea and it's conceivable, you should go ahead and do it. ;) Sounds like a really good feature that would save time. From what I can understand what you mean, forza studio > model > click part > get hex code of said part > quick way of replacing in hex edit?
The way i see things is if it's available to take, on xbox.com will error and say oops!. If it's taken, it will show the old profile with the random generated gamertag. Every available tag i see does this.