If you *need* to preserve the cottered-crank character-- and I can see why one would, the cottered units are very elegant-- any bike kitchen or co-op has a plethora of spindles and cranks lying around for cheap or free. Not many people want to, or even know how to, deal with them. But if you can't get the old ones off easily, don't waste time. Finesse is fine, but sometimes brute force gets the job done with less hassle.

If you can't find new-to-you cottered cranks, they are a glut on eBay.