Check with Hernley's Farm Equipment in Elizabethtown, Pa. (717-367-8867) Owner told me recently that he had a few on hand. Everyone in that area wants an impeller.
When you do get it out - iwould suggest taking to machine shop and see if they could cut anew key way, and i would replace the pulley.....if my memorey is better than my spelling, that chopper assy is about $1K.......it might be worth waiting a dayh or so to get the machine shop the fix you up.