1. You should be able to remove the stuck cotter by supporting the crank arm on a piece of wood or a piece of metal pipe stood on end and driving it out with a hammer if you don't have a proper press. It should not be necessary to saw the spindle and that would be difficult in any case as it is hardened steel. Do not pound on the cotter pin with out supporting the crank arm and frame securely.

2. If you don't have the proper pin spanner, a small punch and light hammer can be used in the holes to turn the cups.

3. A modern (either cup and cone or cartridge) bottom bracket may fit if the threading is standard "English", "Italian" or "French" and if the bottom bracket shell is a standard widthe like 68 or 70 mm. The length of the spindle will be determined by what replacement crank you choose.