It could be as simple as replacing the spindle, but there are a lot of crank sets that would fit once you get the spindle changed, and therein lies the rub.
I've always just fixed the immediate problem first (to a spindle with square taper) and worry about the next step, because it often takes care of itself.
So that's what I'd do, either replace the spindle or the bottom bracket, and go from there. The length of spindle, etc is better handled when you can provide some info and pics.
This is where to get help, though.