A lot depends on the player itself. I have been fooling around with this kind of stuff for a while now and have come up with some considerations. First of all, 6 volt to 12 volt voltage boosters are fairly common and no too expensive anymore. But you also have to switch polarity most of the time. That can be easy or hard depending on the player. Most contemporary electronics come in plastic cases. Being non-conductive, plastic components are easy to mount and easy to wire; just swap the leads. When you get into components with metal cases and chassis, it's a whole new ball game. You either have to swap the polarity in the booster or completely isolate the component.
There are a couple of other complications. The readily available voltage boosters are relatively low-current items. Again, with modern components, most are low current draw themselves so this is not a problem, but who knows what the amperage draw of a 45 year old 8-Track player might be? When you start increasing the amperage capabilities of voltage boosters, their price can quickly get out of hand. Also, most modern stuff tolerates "dirty" power quite well, so there are seldom problems, And, if something breaks breaks or burns out, replacements are usually pretty cheap. It's probably not that way with stuff that's almost 50 years old.
I haven't tried this exact application, but I have done enough similar stuff that I would advise against trying it; there are too many factors working against you. In the final analysis, drawing from my experience with 8-Tracks, if you did get it to work, it would eat your favorite tape the first time through.
To tie a ribbon on it, I don't think Lear even made a 4-Track in a 6 volt version.