Not a whole lot you can do but, is it a four banger or a 258 six? If its a 258 six you can put a head on a 4.0L on it to pick up about 40 horsepower. You will have to make some modifications to do it but, its do able. You can also do fuel injection for either engine but, it can be pricey. What is your current set up? Auto? Manuel? Tires,lift?
In 1990 it should be a 258 straight 6 unless a 4.0 has been swapped in. Lets say it is a carbed 258 and I'll tell you what I did with my 258 for more power. I installed a 470 holley TA 4 barrel carb, offy dual plane intake manifold, roller rockers, comp cam, higher compression pistons, ported head, header, dui distributor and MSD. My 258 will scream, it really bumped the power to well over 200 hp. The 4.0 head works best on a 258 if you go FI. I found a ported 258 head works better with a carb as far as making more power with my other mods. The 258 is capable of making great hp and gobs of low end torque with throwing a few parts at it.
1987 wrangler, 258 I-6, 5-speed.
2007 liberty sport 4X4, automatic
2009 ford ranger 4x4 FX4 OFF/ROAD, 5-speed