The problem would be to travel back in time to ancient Greece.
LOL Good point.
If you want a reasonable facsimile of pankration I would say take up Judo and kickboxing and you would get the general idea of what pankration might be like.
Being partialy Greek I did quite a bit of research on the subject and found these things out:
Pankration translates to "all powers"
Pano=Top or up
Kato=down
They used wrestling style takedowns and throws including many Judo type throws we see today. They used strangles, arm locks, foot locks and knee locks. They did Ground and pound and they used punches and kicks.
There is no mention or drawings of any subs from their backs but I have seen vases with paintings that looked like sweeps. In particular I saw one vase where the man on his back was grabbing the ankle of another man "standing up in his guard" even though the "guard" in and of itself was not a common position if they could help it(IMO).
I would say right now pankration is more or less MMA today and open to interpretation of the instructor.
But if you work Judo throws, wrestling takedowns, Kickboxing and sub from the top, pins and sweeps from the bottom you probably would be very much like one of those ancient greek olympians in terms of skill sets.
Fighters today that might have similar skill sets would be any "well rounded" wrestler that has decent sub ability from the top and escapes from the bottom.
Dan Henderson perhaps, since he will shoot and throw, can strike can get to his feet. Maybe Lidell although he doesnt use subs much.
I would say Fedor but I believe Fedor would have better subs off his back than the typical pankration stylist back in the day.
Help me out here if you can think of better fighters that might have similar attributes to ancient greek pankrationists:
Good strikes on the feet, good wrestling takedowns and throws, subs from the top or from rear mount, good escapes from the "guard" postion. Almost no subs from the guard postion or perhaps only the most basic such from the "guard".