... to transport it back to the late 1960s & have it run on the gas available back then?
I think a modern car would run okay on 1960's gas but the lead in the fuel would destroy the catalytic converter.
It would probably clog the injectors and sensors too. Possibly gum up the valves, depending on the type of fuel injection
So prep work would involve removing the catalytic converters & replacing lead intolerant bits and bobs.
You'd need a stock of consumables: filters, brake pads, modern lubricants etc. oh and coolant.
I'd probably take a set of tires.
A 2021 vehicle would run fine on 60's gas. it would eventually render the catalytic convertors useless. The check engine light would come on, P0420, P0430, but otherwise it may actually run better. There was no 0-20W synthetic oil in 1965, but they did have 10W-30, which would probably be OK.
You couldn't update your entertainment system on wi-fi, but AM/FM would keep working.
Of course, you would be detained by the FBI as a possible space alien. :lol:
I'd take a mid-trim Toyota Highlander back there. Fairly typical family hauler that sells well. Something that people could identify with.