I am not sure how much interference you are talking about but since you say you could beat in but it will not go flush to bottom of keyway on the crank I am going to assume we are not talking about much. They make small diamond files that will cut hardened steel but you would probably not end up with straight sides on the keyway. You would have to keep all the pressure towards the bottom of keyway while filing the sides so not to file the top of the keyway too sloppy. It is doable but not as precision as it should be done. I have also made step keys to fix similar problems. It would work best with a surface grinder or at least a mill though. You just grind the half of the key that fits in keyway taking the same off each side so your centers stay equal. Be sure to not be taking any material off the half that fits in the primary gear. Once again, doing this kind of shit with hand tools is a down and dirty fix at best. If you have a local machine shop it would be a breeze for someone to skim the crank slot until you had a good fit and the sides of the keyway would be nice and straight at the same time. Sounds like a nice build, I would either look in the local machine shop option or try to send it back. EBAY sucks in my opinion, I see some good deals but always seems like the troubles you can run into are not worth the little bit ya save. Good luck and let us know how it turns out for you.
Chip