There is a JT8D-17AR which provides for an automatic thrust increase in case another engine on the airplane fails.
Also, there are the JT8D-200 series engines, which are used on the MD
-80 series airplanes and some hushed 727's.
There is the JT8D-22 engine which is used on the SAAB Viggen Fighter. There is a considerable amount of redesign, especially in the area of materials in the compressor to make the engine usable in a combat airplane.
After that, there is nothing else as the newer generation of engines are more efficient.