I had one apart so while I was there I filed a timing mark on the flywheel with the piston at 1/4 inch past TDC.
This has to be more easier and more accurate than stuffing a screw driver down the spark plug hole.
The one drawback is that the magneto is not fixed. On this particular engine there is not enough play in the bolt holes to matter. But one could always set the flywheel to the mark when working with the coil.