Hi Jason,
I barely know Oklahoma at all (I attended a friend's wedding in Tulsa once and that's the only time I've been to the state), but I understand there is a collection of narrow gauge engines and rolling stock at a Christian summer camp in Adair, OK called Dry Gulch, USA. (Google tells me this is about 2-2.5 hours from Oklahoma City.) They're supposed to have a couple of Porter 0-4-0s as well as (amazingly) a Baldwin 2-6-2 from the Klondike Mines Railway in the Yukon Territory. I don't know if the camp is open to the public but it might be worth a visit since you'll be in the area.
-Philip Marshall