Not to deflect from this forum, but "PlanetIsuzu" also has a lot of carb'd info. From what's posted, I'd start with a carb rebuild kit for the Hitachi, namely a new needle valve and seat. Also, I'd get about 6 feet of vacuum line and replace anything that appears remotely suspect. Bring a chunk of your old stuff to the parts place to get the right size. The EGR valve likes to stick, causing idle issues; that'll stay free with generous applications of WD-40. The air pump pretty much works with the cat-converter, and I assume that's going away, so I'd not worry about the air pump. Along with the usual plug, wire, and dist cap renewal, I'd start with that. Removing and plugging the air rail, etc, are covered elsewhere, but getting it running reliably would be my first concern before digging into de-smogging. Regards!
'87 Isuzu p'up 2.3 gas, 5spd, 4WD, base model, Fed spec