For all the work it would take to get the head off to rebuild them, buy the new valves, seats, springs etc... you could get a different motor and install it in half the time and prob cost a lot less. You can pick up a 4.0L (will fit on your tanny) and fuel pump (fits right in existing tank) for no more than $300. Rent an engine hoist, buy some beer and make a weekend out of it.
The nice thing about these trucks is that parts are a plently and you gan basically go find a ranger with 4L - grab the harness, motor, flywheel, and computer and off you go. No cuting or splicing, no having to cut into the fire wall, of fab engine mounts, plug n play.