I used a modified woodruff key on my own HHB PMA swap, but you are correct in that the new instructions do not call for it, and that the taper fit is more than substantial enough to keep the rotor from slipping under load. The key slot on the shaft is not a match for the key slot on the rotor, so you pretty much have to waste an afternoon filing and fiddling to make a key that works.
Are you sure the rotor has slipped? I know it sounds too obvious, but are you looking at the right mark? Mechanical TDC and the Fire mark are not the same.
If your rotor has slipped, it may be due to incorrect installation. Tapers like that should be scrupulously clean and DRY when installed; any grit, surface corrosion, or lubricant can screw up the security of the fit.
Having said all that, the only thing that would be affected by a slip (I'm not talking about a sloppy loose free-spinning condition, just a one-time little slip) would be the position of the timing mark. Having a key really only serves to make re-establishing the timing easier if you ever had to re & re the rotor for some reason.
As long as everything else is okay, just re-establish TDC with a piston stop or other accurate method, and re-draw your mark on the rotor.
