There are a few tricks that can help loosen old, frozen screws. One thing to try is to heat the screw using a soldering iron. Sometimes the screw is frozen/corroded by varnish/glue/miscellaneous gunk that heat will melt or loosen. Another is to place the screwdriver in the slot, and rap it sharply – the shock can help break the bond. Sometimes tightening the screw (just a bit) will break the bond, after which the screw may back out easily. Of course, make sure the screwdriver is the right size for the slot – not too big, or it will not seat all the way down into the slot, and not to small -- no play allowed – or the screwdriver will cam out and destroy the slot and the head.
If these things do not loosen the screw, you are in for a bit more trouble – so try them first.
Also, the mast step could simply be split away from the screws with a chisel, and a vise grip will grab the screws so you can turn them out. This may be the easiest way -- if you don't want a mast step, there is no loss, since the one you have does not appear to be original. If you do want one, a new mast step is easily fabricated and installed.