Go online & buy a couple sets of new cotter bolts & nuts. Have on hand before starting work. In the meantime, start basting the crank set with penetrating oil or diesel fuel. Unscrew the cotter nut till it's 1/2 way off & tap it with a hammer. If it doesn't come loose no worries. Remove the nut, get a punch, & pound the living crap out of the cotter bolt till it pops out. (Why not - you're not going to re-use it. You got new ones). No big deal. Have fun. Be good.