What mag to you have? It should be a Fairbanks Morse.
The instructions for mag timing are on pages 4-54 thru 4-57. I am assuming you have the governor removed also. That is the only way to get the timing correct when reinstalling the magneto. Pay special attention to paragraph (2) on page 4-56 concerning the impulse coupling.
There are actually (2) timing marks on the cam gear and you need to be sure you time the mag to the lead mark, or the impulse will trip when the piston is halfway down the cylinder, and the engine will start hard or never start.
I found this out last year when I was resurrecting my MEP018 that I had gotten (15) years prior and never did anything with. It now runs well and will provide power to the whole house if needed.