Check the hazard switch location, column = 90spec dash = pre-90, headertank is a give away also, round = 90spec

Splicing the mk3 engine harness to the MK2 fusebox wont be that hard if it is pre-90, you only need to worry about the engine side the lighting part you fit from another MK2 and leave alone. Other option is to fit a complete 90spec MK2 car loom, bit more work but not too bad once dash & carpets are out, tho you need all the fusebox, fusebox hanger and outer column assembly. Easiest way for you is to unwrap the MK3 engine harness and bin everything not linked to the ECU, then get your pre-90 MK2 harness and bin everything that IS connected to the ECU. Once you've done that use my fuisebox faq to figure out what the remaining wires are, and where on the pre-90 plugs they need to go. Or ask on here! Also if you do end up with an ABF pm me your email addy I have ABF ECU wiring diagrams.
Keep the MK2 coil/tci setup though as the MK3 ECU signal wont run the MK2 rev counter, plus the MK3 coils are a bit pants. What you do is chop off the MK3 coil and splice the green/red wire to pin 6 of the TCI-H unit (thats the metal thing bolted to the top of the 16v ecu bracket)
For the fueling side you need to connect push fit abf lines to the threaded k-jet fuel line fittings, what I do is find a pair of under car line -> metering head hoses, you chop the ends out which leaves you with a pait of threaded fittings with a barbed end, push the abf lines on there and clamp with decent fuel pipe clips

Oh and you'll need to fit a lambda probe into your exhaust somehow, a G60 downpipe + cat bypass is a bolt on solution if you can find one, or you can get a suitable lambda boss off ebay and have it welded in.
Other option is to find a k-jet 16v harness and run the abf on that, easier in the short term. (danny i have my bullet proof vest and running shoes on

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I hope all that makes some sence!