Turbo: parts from a turbo Isuzu Impulse can be fit to the 1.8L and made to work, or you could just move up to the 2.0L turbo motor right out of an Impulse. (look for 85-89 I believe) If you want to see some wilder home brew turbo stuff on these motors look at
http://www.hotgemini.com and see what they do to Holden Geminis (another Isuzu brand) down in Australia.
Cam:
http://www.deltacams.com can do performance regrinds and I think there is at least one company that makes an all new performance cam out there somewhere.
Header: pacesetter makes a header, thorley did at least at one point, and probably a couple others. I think you can pick up the pacesetter through
http://www.jcwhitney.com I've seen others show up on ebay every once in a while.
Intake: I think it is Lynx that makes some dual side draft manifolds for Holdens down under, though I don't know if you can get them very easy in the states. I think
http://www.carbs.net has some info about them, so you could call them up and see what they can find. You're looking for a G180 Isuzu motor, but G160 and G200 parts will also fit. Offenhauser made an single carb intake for the LUV with a weber style carb base, but they only show up around here or on ebay. I've also seen one dual carb manifold with Delorto carbs on it for a LUV, but I bet you'll have trouble tracking one down.
Carb: Weber 32/36 DGV (DGEV same with elec choke) is available through
http://www.carbs.net and other places around.
All this stuff will cause you emissions headaches being in California, but if you can figure out how to get around that, you can get some pretty good increases from some fairly simple upgrades.
I've looked over the options and I think the best upgrade would be to get a 2.3L or 2.6L Isuzu motor out of a newer (86-early 90s) Isuzu trooper or pickup. There will be a bit of fabrication work to get it in there, especially if you go to a 5 spd trans at the same time, but the extra power will be very noticable. The fab work won't be as difficult as going to a V6 or V8, and you can get most of it to fit by just mixing/matching parts between the LUV and the donor.