Are you allowed to run a different displacement engine in production class than what was available in the body style, Or are you limited to the production engine?
Yes and Yes. Now that may seem like a strange answer to an either/or question. To be more specific - you can change the displacement of the engine that came with the car but it must be an engine that was available in that model. Example - If you have a Camaro that came with a 327. you can bore stroke that engine to whatever size is possible and in the process maybe change displacement class but remain in the Production category. You could also make the engine smaller if need be to drop down a class or two.
So . . Yes you are allowed to run a different displacement but it must be an engine that came with that car, that year. The key here is that it cannot be an engine swap.
See SCTA 2010 rule book page 71 under Section 5.E near the bottom of the page: "A different displacement size of the same design engine may be used provided it does not constitute an engine swap as defined in Section 4.N."
Also - if per chance you are also considering the GT classes there is a variation on this where engine swaps are allowed in GT as long as the engine came from the same manufacturer. (example - you CAN put a Buick engine in a Corvette in GT)