Could always use blocks and indicator as Gadge says but use heat to straighten it. I don't know how this might affect any heat treating of the crank though.
I'm doubtful about using heat here - these small Briggs cranks will bend cold fairly easily anyway.
Masport used to distribute a crankshaft straightener [it was a required dealer service tool in the 70's] that was basically a bench mountable steel channel base with a centre rail, two V blocks that slid along the rail, a runout indicator pointer and scale, and a small screw press. It worked very well, if the crank bend was not so great as to prevent sump removal.
Gadge why not a steel key? Are the Briggs one designed to shear?
Yes, it is the 'designed weak point', so that if the engine stops very suddenly, the key will shear. Rather than twisting the crank, or wrecking the shaft/flywheel keyways.
As Briggs say in one of their FAQ:
Finally, if a lawn mower or tiller blade hits a rock or curb, the flywheel key can sometimes absorb the damage, reducing repair costs significantly. Always check for damage by removing the flywheel to inspect the key and the keyway, the key's slot on the crankshaft. The soft metal key must eliminate play between the flywheel and crankshaft.
