Such a shame. A bit of machine oil on it before storage would have prevented that. This being said, it all depends on a bunch of things. You can cut the crank .020" if need be. They do make .020" under bearings. I have been told though that goes past its surface hardening.
1. Are the journals already cut .010" under for rods, mains, both, or is it still standard size ?
2. Has it had a mishap like a spun bearing ?
3. If is standard size, and its glass beaded to knock off the rust, grind, and polish, will it clean up at .010" ?
4. Is it straight or bent ?
5. Is crank thrust bearing surface worn ?
6. Get it magnafluxed to look for subsurface defects.
Not sure what a glass beading, shot peening, magnafluxing, straightness check, grinding, oil hole radiusing, and polish will run you, or if a new crank forging would end up cheaper or same cost. If everything above checks out, its probably useable, but you would have to compare costs with a new one to determine if its feasable or not.
Hope this helps
Matt