'97 Ranger 4.0l is an OHV, over head valve, engine, not to be confused with the 4.0l SOHC/OHC, over head cam, that the Explorer got in '97 and Rangers got in 2001.
Have a look here: Tom Morana Racing Engines
And remember you have an OHV not an OHC engine.
No, there isn't alot for the 4.0l OHV or the OHC for that matter, unless you are racing in a 4.0l class any money would be better spent on a V8 conversion.
And just to save money, all Rangers came with a CAI(cold air intake) and headers(mid range), you can add low-range headers, but you lose mid-range power.