If one had a crank arm that was butchered, say from wobbling by a dazed rider with no sense and the tapered region was hacked, then and only then would I attempt a re-file of the taper. Pretty dire circumstances. Taking of equal amounts from four sides is possible by hand, but one would have to really as ONE with the file.
Digging into the spindle beyond the hardened region... I would only with a mill and a set up, then knowing that the remaining spindle is short changed.
In short, measure what you have and go spindle hunting. My first stop would be the oldest local bike shop in town that was adult bike oriented in the past. And take a caliper. Marginal shops are still dropping like flies here, many with no external notice, just one day- poof, gone. A bummer, but one guy walking into a smoke filled shop buying oddball parts on a routine basis is not a business plan.
I walked into another local shop last weekend, an immigrant couple bought the business from the previous owner who found out that an independent bike shop was a high work, low income life. Nice couple, I bought $20 worth of stuff before my children requested we GO. It was right before Christmas, the place should have been hopping... maybe I should leave my card for the asset sale.