Me personally I would go to Napa and buy a sheet of gasket material and cut my own with an exacto knife.
Take it apart, and clean all surfaces. Use some oil base paint to paint mating surfaces. While paint is still wet, use cover to print pattern on gasket material. While gasket material is drying, clean paint off gear box cover. Repair holes in cover. Cut out print, and use transfer punches to locate holes for bolts. Use hollow punch to punch out holes for bolts. See how easy that was?:rotflm: Remember, it doesn't have to be pretty, only functional. You might want till after assembly to use an exacto knife to trim outside of gasket.
An alternative to cutting: lay a piece of gasket material over what it will cover. Tap around the outside and inside edges with the round side of a ball peen hammer. Gasket will be the shape you need. What you cannot tap with the hammer, cut with a knife.
What with all the different types of sealants out there, what would be the best type to seal a gearbox housing? I won't be using a gasket, just the sealant.
Mike