Personally I'd weld it if I couldn't get another cover; Any distortion isn't going to matter much, it's only a lightweight cover and should pull down onto the casing OK once you wind th bolts up.
But if you don't want to weld it, take the cover off, file the hole to a round shape and put a countersunk 5/16" or 8mm machine screw in the hole with some silicone around it, and a nut on the outside of the cover.
cover is all part of the axle case on that one isnt it,has it rusted from inside or out.If its rusted from inside you probably want to be having a look in there to see if its doing any damage,can you put a picture on here of the offending hole
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