you need to use the search function on here. You will find many results about HP/perfomance increases you can do....
Just to list a few...
Open element filter or swiss cheese airbox
larger MAF
BBK TB
Custom Chip
Exhaust
Headers
I beleive there are a few cams out there
electric fans
Supercharger
etc.
and while not the 4.0L thats in your '94 many have done 5.0L swaps (later explorers had the V8 option); that would open up a whole knew world of parts to upgrade as 5.0's are big in the mustang world and support an even larger aftermarket
do some searching and dig up old threads. If you have questions post them in the old thread if it seems relevant or feel free to ask in a new thread. There is alot of good information available from this site, including HP upgrades