Well, if the nut is the correct nut of the "flange nut" type (looks like a nut with a flat washer made into it) then it does not use a lock washer. The wide base of the flange area spreads out the contact over a large enough area that it has tremendous holding ability.
But, it is critical that only a genuine Briggs brand key be used and there has been some recent changes made by Briggs regarding the key and the torque value for tightening the nut.
Tomplum can help me out if I err, but I think there has been issued a new key (black color) for these engines and the torque has been raised up to 110 foot pounds instead of the old 100 Ft Lb value.
It is just as important in how/when you insert the key as the other factors.
First off, the flywheel bore and the crankshaft taper must be absolutely clean and free from any oil, grease, or dirt. Solvent wipe if needed using clean rags.
Second, place the flywheel onto the crankshaft and align the keyways.
Third, insert the key into the aligned keyways and gently tap it down flush with the surface of the flywheel. Tighten nut to 110 foot pounds.

Yes. The new key is PN 796335. No fine crack at the key way in the flywheel?

Tom raises a good point on the fineline cracks, Rusty... yer eyes are prolly gettin like mine - a little blurry on the closeup stuff - get thet big magnifying glass and yer best flashlite fo a visual onceover - if any bad indications, get a rattlecan of crack chk/developer and spray 'er down.... if they is fine hairline cracks anywhere around that keyway groove, they will show up like interstate roads on a rand-mcnally map.....

Well, i called the friend, and went there. He had it running. He used his calibrated torque wrench, and tightened it correctly, and it runs.
the owner was there. He was complaining about the "crappy" Briggs engine, and how it was a piece of junk, and all that! I told him, from the sounds of the mis-treated engine--that he ought to be glad it runs like it does. I think he wasn't impressed by my comments, but--as they say--the truth hurts! I offered him $50 for the tractor, but he refused to sell it. I think he wants to torture it some more.

Must have been the bubble gum hadn't dried yet!