I'm looking into getting a little more horsepower out of my 4cy 99 Amigo.
Any suggestions?

For starters do a full tune up. Oil change/filter. spark plugs/wires, etc. Then either buy or make a new intake. AutoZone sells a universal tube that you can use, along with a cone filter. Then you might try a less restrictive exhaust. Maybe 2.25-2.5 piping with a resonator and a magnaflow muffler to keep it quiet. all this should free about 10-20hp depending on the current condition.