My understanding, and it is limited, is that the fuel injection pump is a strong candidate for causing your problems. The last guy I talked to about this same diesel engine said that he found a place that would rebuild the pump. He had to send it off for the repair, but it ran and ran after that. Perhaps your husband knows of a company that rebuilds diesel injector pumps. In his business, I'd think he would. Besides, small diesel injector pumps aren't that unusual. Besides that Ranger and the Escort that got the same basic engine, in that era there were VW diesels, Mercedes diesels, BMW diesels, and others with small diesel engines.

Something else to check is the original manufacturer of that engine. I think it was Isuzu, but I may be wrong. Sometimes when a company buys a major component like Ford did with that engine, they're less likely to stock it for the long run. Even with my sister's Geo, she had to go to Toyota for a part recently, and the car's a late 90's model -- not that old.

As to manuals, and even parts, the best place I know of for obscure/obsolete stuff is Ebay. If you look for a while, just about anything will show up.