1991 Rangers came with 2.3l 4cyl engine and 2 options for V6s, 3.0l and 4.0l OHV
4.0l OHV has 160HP
You have that now so a fresh 4.0l(rebuild) would be the least expensive way to go.
5.0l(302) V8 is a popular swap, 220HP
Especially those V8's out of 1996-1998 Ford Explorers.
But you are talking about swapping out the engine, transmission, wiring and computer, so not just a matter of changing the engine.
Good read here on what's involved: Ford Ranger V-8 Engine Swap