Today at least, you cannot operate a large aircraft on a Part 91 certificate. Maybe Part 125 as many if not all, the car parts operators do or have done in the past. There have been Part 125 operators with a 91 exemption, but I don't think the FAA is allowing that anymore? Part 121 is the logical way for this to evolve just as UPS did early in their evolution.
Buying an asset that includes a Certificate would seem to be a logical way to go. I believe that Evergreen bought the Johnson Flying Service Part121 certificate back when they first got into the airline business. At the time there had not been a new 121 certificate issued for decades.