Key isn't absolutely necessary. Make (small, exact) marks on the "outside" of the flywheel for exactly correct rotational position. Place flywheel, and tighten flywheel nut at first "gently" and then tighter until flywheel held in exactly correct rotational position and tight on the taper of the crankshaft end. (It is the taper"' that holds the flywheel; the key just lines it up, really.)
Other relevant activities can include printing out a "degree wheel" (from the internet), sticking it onto an old CD. Observing timing as you install flywheel, etc. Top dead center can be observed with a dial indicator, but lots of people are pretty good with a metal pin, etc.
