I have a soft spot for the old AN Mountain. It has no apparent prototype. My impression is that a pristine kit and a well built model are of the same value - say $200-300. That is why you will never see this kit offered commercially again. Same is true of the other AN or Varney O Scale steamers,
If you look hard enough, you will see that Varney supplied 3-rail drivers and wheelsets. That means 3-rail conversion could be trivial - but don't expect O-27 curves without grinding up that beautiful brass cylinder block and trashing the tail beam.
I got one from Downtown Hobbies in Chicago for $35 complete somewhere around 1990. Let me see if I can find a photo or two.
Yes, here are three: The first was a casting I got in 1962 or so, combined with Lobaugh drivers and wood block tender - 1 1/8" gauge. The second was the $35 deal from Chicago. And the last one shows that the model is capable of some artistic changes - I extended the smokebox, modified the cab mount, and added a boxpok main driver - a credible, if not entirely accurate, SP GS-1:
