Remember to disconnect the sparkplug lead first, think aeroplane! Remember to turn it anti-clockwise to remove. A Heavy blow to the bolt head may shift the rust up into thhe thread in the end of the crankshaft. Some versions had an alloy collar or spacer and electrlytic reaction may have bonded the alloy to the steel thread. A heavy blow to the end of the blade with a copper mallet, in the correct direction may unwind the whole lot, especiallly if you can lock up the shaft.
If all fails, visit your local civic amenity tip. A whole lot of good machinnes end up there, a cheap source of parts and engines .I have picked up mowers , allways choose Briggs and Stratton for reliability, I collect these a s and when available, usuaally for £3-£4 complete..