tricky as the syncro has extra wiring for the rear diff n stuff... not really in a position to advise on that part of things, danny or diamon hell should be along shortly to advise hopefully.
given the choice id prefer to 90spec the car, and plug the seat engine loom in, but you would need to reverse-engineer the syncro part to make sure all the diff bits work as they should?
Other option is as i say splice the abf loom into the fusebox using the old engine plug, which isnt that dificult but you might end up paying thru the nose. Print out my fusebox FAQ though and pm me your email addy and i'll send pre & post90 MK2 diagrams + ABF engine loom, print them off too and the sparky should have an easier time of it. Once you strip the engine loom down there arnt many wires that need to be spliced, if you kept as much of the pre-90 engine loom as possible and onyl fit the ECU bits n pieces.
Really you need to have a long conversation with the mechanic, at the end of the day he is the one having to do all the work so you want to keep him happy
Problem with going k-jet is you need the k-jet main pump & fuel lines, metering head, inlets etc plus you still need the enghine bay loom as well, but at least if you find a pre-90 one it'll plug straight in and work. (assuming the syncro bits are integrated with the lighting loom?)
