Yes, M8x20 is the dimension, not more complicated than that. MRX is the designation for the screw type in the image, i.e., machine thread, Phillips head, round head, not countersunk. This type of screw can be found in various places, such as in hardware and building supply stores, so it should be easy to acquire even in small quantities. Some have indeed switched to the Torx variant, but maybe that doesn't matter much?