Unless you have a class racer or other application that mandates stock heads I cannot see any way you can do better than a new set of aluminum heads. Core prices for cast iron heads are low and shipping is high. Plus, Aerohead might, but I haven't run into any head rebuilder that wants to buy extra cores. Got a stack of used cast iron heads in my garage that's worth scrap only.
Porting a stock iron head costs more than the complete aluminum replacement head.
R.