You could hire a private detective that specializes in such things. I did this for my 1956 Corvette and managed to get info on and speak personally to every single owner. Took the guy about 8-9 months to trace all of them (there were 4 I think). I found out so much info from the previous owners it was absolutely worth double what he charged. So much history on the car it was awesome, plus I made one owner really happy being able to see it again.