Well MDS brand engines are not being made anymore. But at the time they were in production there were some parts available. So there might be a backplate out there someplace. MDS parts pop up from time to time on Ebay. The other thing is if the ear for the screw hole didn't break off too much, you can use a washer with the screw to help lock it down better. Maybe a little gasket sealant in the corner too. You can make a new gasket easily enough using a new X-acto knife blade and some card stock (the paper 3x5 cards for example). Another route is to find a junk MDS engine to scavenge parts off of.