I've never seen any reasonable price rebuild kit for the 2.8 but what I have seen is people piecing together what they need, part by part, which takes a ton of time, but turns out great.
But with that said, the general consensus from TRS and the members will be to skip to a 91-94 explorer 4.0. I can see clearly on both sides of the fence. I love keeping the "heart and soul" that a rig was built with in it, because each rig has its own feel. But, when it comes to putting that much work and money into it, the 2.8 is just not the canvas to paint this picture on.
After a rebuild, the 2.8 will be more driveable, way more reliable, have the power close to what it came stock with(even with a more aggressive cam), stop the ungodly blow-by for a while, and quiet down quite a bit. The gas mileage isnt going to change as drastically as you expect, the power is pretty lacking even in perfect shape, the feedback carb is a bastard to work with, and the cost is just not worth it.
I don't want to be all negative, I personally have a 2.8 in my BroncoII, and it does have a neat feel. But with a meager 127k well cared for miles on it, it sounds like its going to explode, it smokes like a chimney, leaks all over itself, blow-by ruins a good filter in 10 minutes(i run my crankcase breather out a fender now), hesitates, stalls almost every time it returns to idle, carb banks out and it dies going up hill, down hill, side hill, its just not the motor for a rig that sees any offroad or onroad action....
If you have 300-500 that you are willing to spend, look around on craigslist for a 4.0. Fuel injection really is a great thing, and a 4.0 is perfect for our rigs, it feels right, because that's what they are made for.
I have 2 questions for you though. 1: What are you looking to spend? 2: What are you expecting out of that rebuild?
Up to you though my man. That motor may have some more meaningful value to you, that makes it worthwhile.