I believe what you're looking for is actually in the R68 by Mr. Railfan. Even though it's supposed to be the R68, the exterior has the side of a R68A.
Like bve said, you could stick that in the R68A folder, but remember to drag the exterior folder and the extensions.cfg as well. Also, remember that you're modifying this for your own personal use, so DO NOT distribute what you've made unless you've gotten permission from both developers.