I did the conversion a few years ago after blowing a head gasket. The parts from a 91-95 Wrangler or Grand Cherokee will bolt right up, though a few mods were needed here and there.
A 4.0 head will bolt on, though a few water passages need to be closed off, which is easily done with JB Weld.
Not only does the later head work better, it fits. If you re-use the 4.2 head, the intake and exhaust manifolds won't fit quite right and you'll be jury-rigging attachment bolts, gaskets, etc.
Also, the 4.0 head is designed for MPFI, and has intake angles that make much better use of the MPFI itself. That advantage is lost with the old head.
There are several different castings available, two of which are much better than the others and those casting numbers are posted in several threads.
Having done the MPFI conversion and driven it for several years, I can't recommend it highly enough.
You'll need a wiring harness for the new set-up. You can either strip out an OEM one, or bit the bullet and get one from HESCO.
After all I learned doing this the first time, in future I'd just modify an OEM harness.
Someone else has already commented on the TPS situation. I went with the HESCO kit, but again, knowing what I know now, if I ever do it again I'd go with the flywheel option.
But check out the threads on this already posted around here; there's more info than you'll ever need.
"We are all of us in the gutter...
Some of us are looking at the stars."
~ Oscar Wilde via Chrissie Hynde