wiring wise, use a mk2 scirocco loom (out of any 1.8i model)
ALL the plugs fit mk1 stuff fine - the dash, lights, everything. goes straight in.
and use the engine loom from the mk2 16v
i did this on a mk1 once years back. it all worked first time. easier than you think!!
the air box takes a little jiggery pokery to get it to sit right, but that was it
oh and use the mk2 brake servo. the mk1 servo wont fit.
use the complete mk2 injection system. it will all work fine.
and get a 4 branch manifold, like has been said already.
that's about it. it's a pretty straightforward job to do.
once it's done, take the arch kit off that mk2 and bung it in a woodchipper . . . .