RELAX, Relax, relax...

This is not a big problem. It is probably a 5/16" National Coarse Thread.

Just look around and find a bolt that fits.

Once you find one you can screw in easily with your fingers, use a little rod to see how deep the hole is.

You want a bolt that is a little shorter than the depth of the hole PLUS the thickness of the oil pump foot.

Lastly you want to use a grade five bolt or tougher... (grade 8).

Grade 5 would be a bolt with three hashes on the top.