I will see what I can come up with for you on Sunday. I'm sure Modelhouse has some resin promo parts for the GTX that you could use on the Roadrunner.
Another option would be to search evilbay for a broke or damaged promo and use it to convert the ertl 68 Roadrunner. I wouldn't recommend using a nice promo since they go for big money but you can sometimes find one for parts at a reasonable price. This is how most resin kits and parts are made.