Dear Mike,

I would suggest looking for a used main board on ebay/other sources. It should be cheaper and less time consuming than swapping out the controller for a replacement lower cost one. Do you have all the backups available?
Are you sure the main board is dead? What is the error shown on the main board?

In India there are some very good repair companies which do the job at very reasonable prices (One such company I have used the service of is GK Technologies in Bangalore - Can recommend their work). We also get used boards at good prices on ebay and locally here. I would recommend this as the Fanuc 18M is generally quite reliable from what I understand.

Best of luck!

Jaideep