you need to swap lots....first the engine with the loom, then the driveshafts and gearbox with hydraulic clutch (unless you fit a seat ibiza box), the ecu and steering columm if post 95 as this controls the immobiliser, then you need to fit better brakes and its wise to fit gti suspension (the 1.8 has smaller driveshafts,control arms and has oil shockers!)
Not simple really!! It would be cheaper to buy a proper 16v
