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Any of the stock Ford Ranger motors should fit (2.3 4cyl, 2.5 4cyl, 2.6/ 2.8/ 2.9/ 3.0 and 4.0 V6), it just depends on how much work you want to put into it. The 3.0 uses a different bellhousing bolt pattern from the other V6's, but is very plentiful. The 4.0 has lots of power, but you'll need to replace your entire drivetrain to handle it. Any motor you pick will need a lot of donor parts like manifolds, computer, wiring harness, & possibly motor mounts. There's certainly a lot to consider. If you're serious about power, there are kits to drop in a 302 V8, including the custom radiator, motor mounts, headers, transmission crossmembers, etc.
With fuel costs being what they are, the 2.3L is the way to go right now--yeah, it takes rpms to get moving, but any engine does anyway. You get better city mileage than any other engine gets with hwy ratings. Besides, odds are you don't have that truck because you can afford Ferrari gas mileage.
Your model had a 2.9L v6 option and the 4.0L will use some of the same structural parts in a swap.
So, really, if you have to ask, don't bother...just buy a truck with what you want in it.