The most common screw used to mount .40 engines to engine mounts is called a socket head cap screw. Size 6-32 is good for this size engine. It is a machine screw (not a wood screw or sheet metal screw). The head is designed to be screwed in with an allen wrench. You drill the holes through the mount so that the screws are just tight. The screws can cut their own threads through the plastic mount or you can tap the holes with a 6-32 tap. Or you can drill the holes even larger and put a nut on the back side. Use a locking nut - the kind with a plastic ring embedded in the nut that keeps it from vibrating off.