There is several ways you can go by using all Isuzu parts:
You can buy a rebuild kit from a local auto parts store or on ebay and rebuild it yourself. (Not my personal choice BTW.)
You can find another 1.8 G180 out of a Luv, Imark , or a Pup and just put that in. It's by far the simplest, often times cheapest and easiest solution - and everything will bolt right up just like it does now.
You can upgrade to a 1.9 G200 out of a newer Pup or a factory hp 1.9 G200z from an Impulse that comes stock with taller pistons and stiffer valve springs and a slightly larger cam - and either one will bolt right in. Neither 1.9 version has the air injection nozzles built in to the head that the 1.8 does though - so that will have to be worked out if you have to use all of your stock smog stuff.
But installing your old 1.8 G180 head on a 1.9 G200 block will also boost up your compression a bit and look just like a stock setup. Just don't try to use too big of a high performance cam when installing a 1.8 G180 head on the Impulse 1.9 G200z engine or you will have serious major valve-to-piston clearance problems due to it's taller flat-top pistons. And you will probably need to run premium gas in it to keep it from pinging like crazy under a load.
(But IMHO - it sure is worth it for the increased hp/torque you get.)
You can use a newer 2.6 (which is another Isuzu 4 cylinder engine BTW) with an older 2.3 carb intake/distributor/ignition and it will pretty much bolt right in. (As far as how hard the ignition/distributor is to wire up - I'm in the process of figuring all of that out to do this to my truck.)
With either of those combinations, you can bolt a newer Isuzu 5 speed right in and still use your stock rear-end, or switch to a newer heavier Pup rear-end that will easily bolt in without too much fuss too.
Swapping out the 4speed for a 5speed is pretty simple, but you will need the front yoke from the truck you got the 5speed from's driveline because the spline count on the tail of the 4speed and the 5speed are different.
Swapping the rear-end for a newer one just requires you to use your old stock leaf springs (to get the wheels to line up correctly in the center of the rear wheel wells) and to relocate the little tab/bracket that the rear brake line junction box bolts to. And then you will also need the rear yoke from the donor truck's driveline too. The u-joints, size, and length for both drivelines are both identical - but the bolt pattern for the yoke connection on the newer rear-end has a different bolt hole spacing than the stock one.
Then add a cam, a header, an optional 50 amp alternator, an electronic distributor/ignition module, etc. - and you will have a light and pretty fast 4-banger truck that still gets reasonably good gas mileage.
You can check these threads out for more info too (and do a search for even more):
http://forums.luvtruck.com/viewtopic.php?t=12797&highlight=impulse
http://forums.luvtruck.com/viewtopic.php?t=10702
http://forums.luvtruck.com/viewtopic.php?t=14408&highlight=isuzu
I hope all this helps ya...good luck with your truck!