I need a jet engine. It must have been comparable with the J79 in service life and about 3/4 the diameter. Any ideas?
Comparable in what way? Time of service? Cost? Fuel efficiency? Thrust?
If it's thrust, you're pretty much out of luck. Thrust is governed by mass flow and for similar performance you need similar mass flows. Mechanically things that hold the engine together require a certain mount of space at the centreline of the engine and military engines have always pushed the packaging of their engines as tight as possible so the minimum diameters for a given airflow have not changed much over the years. And a more modern turbo fan, while providing more thrust at lower engine mass, usually does so by increasing the diameter to accommodate the low (or high) bypass duct of the engine.
And, because the mechanical gubbins of an engine doesn't really scale well with the airflow, even much lower performance engines don't really get much smaller than the old school turbojets. The 38" diameter of the J79 is really hard to beat unless you're willing to go down to really small jets in the 4-7,000 lb thrust category.
Paul