Unless the owner has some documentation or the car has been been discussed in detail on some forums, the only sure way is to contact Joel Rosen himself. Rosen, as you probably know, was the principle owner of Motion Performance. He has retained all of the sales records of Motion so he can verify the car as original or not, along with the original options and build specs. He is a hard businessman, so be prepared to pay.
BTW, Motion Performance shop was about 5 miles from my house back in the day. We younger guys used to go and watch them test their cars on their “test track” which was Sunrise Highway, right in front of their shop! Although the were most associated with Chevrolet, through their Baldwin Chevy dealership relationship, they did work on all makes, so it was not unusual to see some Oldsmobiles, Mopars and the odd Ford back in the day. They even sponsored a 1965 or 66 Mustang drag car with a 289 bottom end and Boss 302 heads, called the Hugger Mugger. It was sold at auction a year or so back by the second owner for well over $100k.