Looks like you have over 1/2 of the hole there, my suggestion is to get a grade 8/"AN" washer and trim it to fit the casting before you put the bolt/lock washer back in the case.
"AN" washers are the same thickness for better load under the bolt head when you torque the bolt, it can also be welded using a carbon arc rod in the hole for a temporary plug while welding it using a Stainless Steel welding rod on an aluminum plate, then all you do is knock out the carbon plug and chase the hole with a drill bit.
My .02 worth'
Redrockman