Some prior owner installed a push button start that requires the key be in the on position before depressing. However, it has degenerated to the point that I am not able to turn the key back, at least not until I get lucky. Any suggestions? I'd really like a decent replacement. Is this a part that can be found still? Or is used my only option? Is it easy to replace and rekey?

Used is your only option and good luck with that. Those buggers are hard to come by. When you do get one you may not have to rekey. They tend to be worn enough that your old key will work. I replaced the ignition on my '85 Brat and use my old key with no probs.  

They pop up every once in awhile. You could try a yard, but they can be time consuming to pull, and there is no guarantee it will work properly.

You could pull the barrel (the part the key slides into), remove the "u" shaped pins from the barrel, and reinstall the barrel. That will make any key from that gen fit but may take away the binding you are getting.

Believe it or not, I took it to my mechanic who was able to find a brand new one! And, it now has a new Subaru key too. It wasn't a $50 job, but it's not like I was going to be able to do it myself anyway. Virginia, there IS a santa claus.