Since your alternator will excite at lower PRM's with power jumpered the the #1 "switched excite" terminal, you can run a wire from the #1 terminal to the ignition switch if it's distributor ignition. Use a diode or an "idiot light" in the lead to the ignition switch to prevent current feedback, and engine "run-on" when the ignition is shut off. A 1N5008 diode works GREAT for this application. The cathode end (with a white band) goes toward the #1 terminal. 
With the simple wiring harness as shown, you would not even have needed a (silly) self-exciting alternator. (The diode is shown uncovered for illustrative purposes, only. Before installation, I cover them with 2 layers of heat shrink tubing, to prevent the leads from flexing and breaking, due to vibration.) Use a momentary "ON" pushbutton switch, or an oil pressure switch as in one of BobM's diagrams, if it has a magneto.
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