How Can a bent lawn mower crank shaft blade mount be straightened?

Short answer - it can't be straightened (safely).

Heating it enough to bend it straight, would heat up the bearing/seal and destroy it, resulting in oil leaks (4 stroke) or crankcase pressure leaks (2 stroke). If it is just the pressed on mounting boss, heating will expand it, and it may not lock onto the shaft again.

All components linked to the blade are critical parts. The blade tips are doing around 200 mph / 320 kph when running at speed. The inertia forces are absolutely huge at these speeds, and any loss of strength through heating, hammering or bending can result in catastrophic failure. Knowing how they build lawnmower chassis these days, would you trust one to contain the shrapnel?