Imported British Car proof of ownership


I have been looking at a vintage British car that was imported about 15 years ago and has been through two owners since then. The current owner has not registered it and the only paperwork he is able to show me is two insurance cards (Yes, from Hagerty), from the previous owner in Illinois, membership documents for a British roadside assistance company, and a window sticker from the garage it had been sold from in the UK.

What should there be? Would Illinois have issued a title or a transferable registration on something this old? It’s a '63. What would the current seller in Pennsylvania have needed from him to register the car in his state?

Thanks for any information,