The best 4-banger combo for a Luv is a 84-86 1.9 Impulse engine (G200Z), re-worked cam from Delta, 82 isuzu pup/isuzu imark/luv electronic distributor and ignition module, weber w/offy intake, header, 86-89 isuzu pup 5 speed w/rear end and driveline (from a pup with a 2.6), and IMHO a step-side bed.
The distributor/ignition module conversion is a very simple wiring job and instructions are available here on this site. It greatly increases you hp, mileage, and used in combination with an accel high-output coil that costs around $35.00, this all will give you the best bang for your buck.
I also personally ran 2 1/2 inch pipe out to the back with a cherry bomb, a cheapo auto parts store muffler, and a cheap resonator tip. It sounds quiet enough when you are cruising by the cops, but has a nice rumble when you put your foot into it. Plus it's a cheaper way to go and the back pressure is perfect for this engine combo.
The only adapting work necessary on the tranny and rear-end swap is you will need to swap your springs to the pup rear-end to keep your wheels centered in the rear wheel-wells, and you will have to relocate the tab welded to the pup rear-end that the brake line junction box bolts to from one end to another.
The same flywheel, pressure plate, throw-out bearing, pilot bearing, rear engine adapter plate, and clutch disk will all still fit and be useable.
You will also need to use the pup driveline because the splines of a stock 4 speed and a pup 5 speed are different at the rear of the tranny where the driveline connects, and the bolt pattern of the two drivelines are different where the tail-end of the driveline connects to the rear-end yoke, but the u-joints are all exactly the same.
Just find a pup with a 2.6 to get your rear-end, 5 speed, and driveline from to make sure you get their heavier version, the swap will go in quick and easy, it will last you a long time, and your speedo should be right on the money.
Now to help you on your search...use this site:
http://car-part.com/
Have fun and happy hunting!