One of the best sites is:
TransAtlantic is located in White Marsh, VA (SE on the bay). They are the ones to go to related to Perkins anything as well as other engines. They can do a rebuild or sell you a rebuilt engine. having a rebuilt done can be quite a bit more affordable. Also replacement may require more work than you might be ready to bite off. A rebuild would be cheaper and drop back in place. A decent engine survey will tell if you even need a rebuild.
A 1979 engine should not need replacement unless it was poorly maintained. It probably could be rebuilt too if need be.
They can be reached on the phone or EMail and I would rely on any information they give you. I would call them and discuss it with them. They are pretty good folks. I live 5 miles from them and they do a lot of work on gensets and big engines too.