motherboard replacement compatibility for iMacs

Hello,

I live on seaside, and accepted that I have to replace my computers motherboards once a year because of damages caused by humidity and salt.

Last broken one was just refused by AppleCare. It is a mid-2011 A1311, MC309 LL/A.

I can't find a replacement motherboard for a reasonable price.

I am OK with the idea of downgrade it (as I did wit my 2010 macbook), but unsure about internal connexions and CPU compatibility.

Will a late-2009 MC413LL/A motherboard fit into ?

What other model will be compatible ?

Thanks.