I guess sheet metal screws are OK for some things, but you will never find them in full scale aircraft, civilian or military, in medical equipment or any high quality product of any type. They are the mechanical equivalent of chewing gum and duct tape! I was once told by a maker of engine mounts that the reason they furnish them is because most modelers will not have the proper tools to drill and tap them. Such tools are cheap compared to the price of a good engine and airplane, and will insure the proper and safe mounting. Some R/C suppliers have the proper drill and tap set, but they are also readily available at any good hardware store.
Generally, the tapped holes, with lockwashers under the screw head, will hold all engines, but if any doubt exists, a locknut can be added under the mount arm. Luck, Eloy.