Install a DX with 1.5 wiring

you can do so yes, theres not much to add wirign wise for the kjet injection system. what fusebox is it, the main thing you need to sort is the fuel pump wirign side, how you go about that depends on what fusebox it is and what electrical tools you have available to you

 

81 will be the earlier ceramic type fusebox, the DX loom may be for the later CE1 type fusebox but really it doesnt make any difference for the engien control side. so first if you split the plastic tubing off the DX loom, carefully take all the wires out for the 5th injector, thermoswitch, WUR and AAV valves you'll note non of it goes to the fusebox, as you pull it out tape it together every few cm or so then it will keep its shape.

heres a direct link to the ceramic fusebox section of my fusebox faq:
https://clubgti.com/forums/index.php?threads/fusebox-faq.219775/#post-1945620

The next potential issue is the ignition system, you may find the old engine is running points, the DX engine will be electronic ignition. 2 options here swap the 1.5 dizzy over and tweak the timing a bit, or convert the car to electronic ignition. To convert it to electronic ignition first check the coil power wire, if its black great just drop the DX ignition control unit and loom in the bay, finished. If its purple/white then its a resistance wire, this is no good. You need to run a fresh wire to the coil from the fusebox, add this on top of the kjet loom you split off earlier. Also if its got the resistance wire in note theres a black wire from coil to 2nd terminal 15a spade on the starter motor, you wont need this any more so it can be removed.

once happy with the kjet engien loom tape it up neat and choose a spare bulkhead grommet to pull the loom thru, leaving the old 1.5 loom as it is. if you need to run a fresh wire for the coil power, put that to G spade 3 or if you have a terminal removal tool you can pull outn the old coil power and swap your new wire to the fusebox pin. leave the original coil negative wire on for the rev counter. Note there are some spade plugs for the various kjet feeds, we'll get to this later.

for the fuel pump side, you'll still need one for the webers so the work you do for this can be left in place once swapped over. For starters get yourself a proper kjet 8v fuel relay, mk1 or mk2. Once you get hold of one note the terminal sizes, the ceramic fusebox ones with fuse built in use 6.3mm spades and the later ce1 relay without fuse has 9.53mm spades. relay choice determines what relay holder you need, and if you also need an extra inline fuse or not.

wire it like so:
30 - permanent live, via fuse if required
31b - rev counter trigger, tap this to the coil negative red/black wire to fusebox
85 or 31 - earth
86 - ignition live (G/3 is fine)
87 - to fuel pump and a male spade

finally for the kjet spades, the red/black needs to be starter cranking live. you may find a spare spade behind fusebox on the main starter live red/black wire, if not you could run this straight to the 15a terminal on the starter I mentioned earlier. the green/white spade can be ignored, its fitted inlien with the wire from thermoswitch to 5th injector and is used on some variants of the kjet system. the red/white spade forthe WUR and AAV needs to go to the spade you put on your fuel pump/relay loom above. finally any brown wires are earths, pop them to the claw above the fusebox