Rudder blades are $215 ea, and castings are $297 ea from murrays.com
I think you will need to get longer tubes for the tiller arms and different grudgons for the transom. The kit from Murray's is about 175.00. And, you can order direct from Performance to 714-835-6416. Also, make sure rudders are pre-drilled.
Now that I think about it you may want to inquire about upgrading to the Inter/NACRA 580 style system
