If you mean what engine with more power will bolt to your transmission, there is only 1.
You have a 4.0l OHV engine with 160HP, the newer 4.0l SOHC engine has 207 HP
These have a similar block/bell housing bolt pattern, SOHC has one extra hole I think.
You still need to swap Computer and engine wiring harness.
AND 4.0l SOHC only came with PATS computers and OBD II, so you will need to add a "wand" on your ignition key slot and an O2 sensor behind the Cat converter.
Your '91 should have a VSS(vehicle speed sensor) on the trans, but have a look, you will see where speedo cable comes out and there should be a wire connector on the same place, computer needs that.
Hopefully you have a manual trans, if not then that may be another can of worms.
I have read a couple of people suggesting the 4.0l OHV computer could run a 4.0l SOHC, I am sure it could "run it" since it is a MAF system but I doubt you would be happy with results.
All in all, you would probably be better of just doing the more common explorer V8 swap, engine and tranny.
A little more work, but more power and lots of how-to's for that
Last edited by RonD; 01-30-2015 at 08:32 PM.