The two aren't related. My guess about the ABS light is you don't have the 4 wheel speed sensors hooked up which I assume would render the system useless. The system calculates each wheel speed to calculate when to activate the system (wheel slip). Since You probbaly also did not install the ABS solenoid and the full braking system from the 2nd gen, you should probabaly just remove the bulb from the dash or cut the wire. You basically don't have ABS. (good because the 2nd gen ABS is very finciky and sensitive so you won't miss it).

About the CEL. I would assume you did not install every single little sensor the ECM is looking for, maybe you need a VSS (Vehicle Speed Sensor) in the auto trans? Or wait you have manual trans. Did you set up all 4 02 sensors? You should get the "codes" stored in the ECM pulled by a diagnostic computer to find out what the problem is. Did you install the diagnoistic connector on the dash? You can find out what the computer is looking for by looking up the stored codes.

You might also look at getting your ECM reprogrammed if any such software or companies exist for Isuzu ECMs in order to delete certain error code triggers if installing the related systems is impossible or too much work. This may be absolutely necessary if you cannot pass emissions testing when your CEL is illuminated. See if you can restore the stock system as a whole first though. Post back when you get the codes read. Might be a good idea to reset the computer by disconnecting and reconnecting the battery first to clear any codes you may have set by messing with wiring, starting the engine and installing the system in the amigo. (Simply driving and running out of gas will set a misfire code that will be stored!) So it would be best to "start fresh" and get all the codes now that exhaust etc. is installed.