Hey Sean, I've read that 50/50 auto trans fluid and acetone makes a great home-brew WD-40 alternative that's supposed to be as good as the best store bought stuff.
One trick I learnt many moons ago for similar situations is to clean the bolt and tig a nut onto the offending bolt. The new nut gives a good head to get a spanner onto and the heat from the tig will heat up the bolt and help break the bond between the bolt and cutting deck.
Good luck
