I find that weird than almost every mower will allow you to do that when a simple relay would prevent it .
Now back to the OP
Tapers are easy to damage so get some very fine lapping paste ( I use Brasso ) and lap the flywheel to the taper .
Keep on going till it is nice & shinny
What do you mean by tapers? You mean the Sledge style tapered press fit of the crankshaft into the matching shape on the flywheel?.
I don't worry about those getting damaged at all. It makes little difference. Lol
Just stick it on and hold it in the right place and tighten it up and actually it'll be fine.
Those keys aren't even there to stop it from spinning but rather just to line it up during the manufacturing process.
At least this is 95% the case.
Also, you can just line it up and get something to drive down into there while you're holding it.
A nail, a screw with the head cut off, a cotter pin shortened a little bit.
It don't matter!
I just put a new Briggs push mower engine back together yesterday with a couple pieces of cotter key in there because I didn't really feel like taking the flagpole off of another one to scavenge an actual Briggs key.