Make one if you can't find one locally. Get a bit of gasket paper from your local motor factor (or agricultural machinery dealer) - the thickness isn't critical. Spray the alloy housing with some wd 40 or similar and press onto the gasket paper. Cut out with a modelling knife or scissors. To do the holes try putting the rough gasket on the engine block where it will go and put a ball bearing on top which is a bit bigger than the hole and then hit with a hammer. Will cut a perfect circle.