Typically, the washer on the front of the prop is flat on both sides.

As for the hub behind the prop, I've seen them flat on the magneto side and grooved on the prop side to keep it from slipping.

Just make your prop hub so that it will fit flush against the magneto and it will stay put when you tighten the prop nut down.

The magneto is usually on a keyed and tapered shaft----it's not going anywhere. All the prop hub is for; is basically a long spacer. Once you line the magneto up on the keyway and put the prop hub on-- then the prop and tighten it all up-- it should stay put.

You could drill a couple of holes through the prop hub. Drill from the top down-- straight into the magneto. Drill into the magneto. Just drill two holes into the hub/magneto and tap them for set screws. Then you can run your set screws into the magneto. That would keep the hub from slipping-- if your really worried about it.

I would probably avoid filing a flat spot on the crank. Might weaken it or throw the balance off.

I'm no machinist- but thats how most of my conversion parts look. I've ordered all my parts from other machinists.