I replaced mine with bolts from Home Depot. It has been so long that I wouldn't gather a guess as to their size. If you have none around... any decent hardware store should do.
Hardware store bolts here as well. You might have to buy a few bolts of different sizes to find the right thread. The place I went to let me take a few bolts outside to my car to try 'em out.
Make sure to match the correct thread pitch.Also,the bolt tensile strength is listed as 9.8.Don't use anything with a much lower number or it's liable to start shifting around one day.
Btw,do you have the black painted Steeda brace that connects to both the sides and at the firewall? You shouldn't have to remove it just to change your oil.I have the said one above and it passes right over the oil cap.I just use a narrow funnel with the bottom cut out wider.No need to remove the brace to add oil.