I think AGCO might be the only source for a new pump. I'd think you would be able to find a good used one, the 66 through 880 all used the same oil pump to the best of my knowledge, and should be interchangeable. O'Brien County Implement is bound to have one, or get you a price on a new one. I'd also think that a good welder could get yours fixed up ok. All the bolt has to do is keep the pump from falling out.

The flange is supposed to seat on the block, so something must not have been right for that to happen. Does it have the right oil pmp in it now? I don't know where a wrong one would come from, maybe a non-Oliver Waukesha.