This is a frequent topic actually. Yes, there are some things you can do. It all comes down to getting more air/fuel in, and getting more exhaust out. Most of the aftermarket parts available out there are for the 76 and newer style motor though, so if you have a 75 or older you'll have to look harder for some parts.
On the intake side:
Get a Weber 32/36 DGEV or DGV carb on there. Besides a near 10% boost to power, you will likely see a bump in mileage too.
Offenhauser Intake manifold:
http://www.summitracing.com lists them still, and they show up on ebay every now and then. The weber will bolt to them with no adapter (stock carb won't so a weber should go with this) and the manifold will flow more air.
Port/polish job on the manifold/head: It can be done. I don't know if there is enough meat around the valves to get anything bigger in there though. No one that I have seen has really tried though. Just cleaning up the ports and chamber would certain be a help though.
On the exhaust side:
Header: Just get one, it is well worth it. Pacesetter still makes and sells a new one, find them through JC Whitney. Thorley used to make a tri- design, though finding them is tough.
http://www.calmini.com sells one just like the Thorley that is designed for newer Isuzu 4 bangers. The bolt pattern is the same but the runners are round instead of squarish.
Upgrade the exhaust system: 2"-2 1/4" pipe all the way back, better flowing muffler. You get more power and a better sound. Much bigger pipe than that won't gain you much though, and can drop the power.
On both sides: Cam upgrades
http://www.deltacams.com does regrinds to get better power. They take your cam and grind a different profile onto it. If you don't ask for super custom it is pretty cheap, but they can custom grind for a specific RPM range if you like.
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95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good
