You can buy the Junior Motometers at larger Model A suppliers (Mac's), just make sure that you get the plain Boyce dial/face plate rather then the Ford logo....You will also need a radiator cap for the motometer, Model A has a locking hinged (flip top) cap for the Junior size motometer, problem is that the model a caps are for 2 1/4" and your M-2 radiator neck has 2" threads, this can be remedied with a threaded adapter bushing...I do have an extra threaded bushing that I machined a coupla years back if you want to go that route...If you want to spruce it up even more get a set of wings to go between the motometer and cap....Me not proud,one stop shopping, Ha
This is a poor youtube walk around of my little M-2 coupe, you can get an idea of what that set-up looks like...... https://www.youtube.com/watch?v=6cDGAY8UrNA