Jon,
It looks to me as if the work was done with a hand file, followed up with progressively finer grades of emery cloth in strip form, used as if you were buffing a shoe. It was not highly polished, as there are obvious tool marks around the rounded edge. The longer edges along the inner part of the chain ring show some irregularity, and the inner radius of the spider, closest to the axle, is not modified, as the file or emery cloth would have bit into the arm at that point. I'm not sure why he didn't radius the two edges of the spider closest to the arm, as that would actually provide some benefit by reducing the spiders tendency to crack there. Possibly he was using a power tool (small belt sander) that wouldn't fit into that space. I would go ahead and try it with a 42 tooth chainring, but first I would bolt it up and scribe the intersections with the spider, then remove it and hand file with a small half round file close to your limit marks, then finish with emery cloth and possibly scotchbrite in progressively finer grades. Darken the surface around the intersection with a black marker before bolting up and scribing to give enhanced contrast (home shop machinists blue.) As the chainring is on the backside, any mismatch wouldn't be very noticeable. Re-anodize the ring (or not) after you get the desired contour. Maybe you should do this first with a French (TA?) replacement ring to see how it comes out before sacrificing a NR or SR ring.
Davis Jensen
Sunny Lomita, CA