Re the hammer thing: a brass drift or one made of hardwood is recommended, especially if its your first go and you don't expect to get a press. The problem will be that of smashing the threads on the cotter pin, rendering it scrap. A Brass drift is usually a bit softer which helps in preventing damage to the threads.

Blocking the crank arm is also essential- there are often a nice set of bearings, BB axle and bearing cups that don't deserve the damage a hammer blow can bring. Block the arm in such a way that the finish of the arm is intact once the blow has been administered!

For this reason, if in a pinch, I would try a 6" C-clamp and a nut from a large bolt such that you fit the nut over the head of the cotter pin to provide space for it to move while applying pressure to the threaded end with the C-clamp in the same way as a cotter press (which is essentially a specialized C-clamp anyway). You might want to use a large crescent wrench to convince the threads of the C-clamp to turn.

Quite often corrosion is an issue- if this is the case and efforts with a C-clamp have led no-where, a shot from can of Kroil (Penetrating-Lubricating Oils) can be really helpful. The stuff stinks IMO, so I prefer to use it outside, but its about 1000x more effective than WD-40 and about 100x more effective than Liquid Wrench. You do have to give it time to creep into the area you want to loosen though. In a cotter pin, perhaps about 3 minutes. Since there is some possibility that the cotter pins were filed (and so have been rusting) upon initial installation, Kroil can really save the day. A small amount of heat can be judiciously applied- keeping in mind that a vintage paint finish is only inches away.

FWIW dept: a very respected mechanic in this neck of the woods (St. Paul) is Steve Hamel, and I am sure some of you on this forum know who he is (even though he is more likely to be working on a 1937 Miller race car or a '47 Vincent). Kroil is a thing that is never missing in his shop.