Wouldn't try it on the lawnmower. Having the blade come apart that close to my feet is a scary thing. Ihad a mower drop a blade once and it missed my foot by 2 inches.

Seriously:Inspect the hub carefully. If there are no stress cracks, and the prop tracks true, trim the tips and balance the prop carefully. if you are really confident, bolt it to the motor and ground-test it in a safe location. Stay behind the plane and make sure that anyone around also is behind the plane. Run it through the entire range of engine speeds and make sure it does not hit a destructive resonance. Be ready to chop power instantly. When it survives the ground test-go fly.
Land carefully and Don't bounce it off the ground again :-)