First off, none of these mods are exactly legal. That said, they will usually make the 4.2L run much better & emissions systems in these old heeps aren't always working the way they are supposed to. So these mods while good, may not pass an emissions inspection. It pays to know your inspector's favorite lunch preferences.
(1) google "The Nutter Bypass". This is the cheapest of the three mods, & can be done without the other two.
There is a picture version of it on this board, use the search feature.
(2) The TeamRush ignition upgrade,
will allow your engine to burn the increased amount of fuel that the next mod will deliver.
(3) Google the MC2100 Motorcraft carburetor upgrade. Send me a PM if you need more info & cant find it.
It pays to do a lot of research on these, before doing them. Before these, I was about to give up on my 90 YJ.
Two years later, I have done several inexpensive upgrades. The two I'm working on right now are the York on board air compressor, & a second monster alternator to use as an on board welder.
The 4.2L YJ lends itself to many Junkyard upgrades for people like you and I, who are on a limited budget.
The three mods I listed, compliment each other nicely & will wake up that old motor. I can shift at 3500 rpms all day, & can take it to 4 grand if need be, although these motors weren't designed to revved much over 3700 rpm's & won't last long over 4500rpm's. If you can get it to run that high.