the only thing you need to worry about is the exhaust manifold/downpipe. You need a MK1 conversion kit! There are a few manifolds ready made for this conversion, or you can make your own by chopping up the corrado downpipe and welding it to the MK1 item. Then you got the fuel lines, if its a GTI you can get away with bending the metal pipes round carefully to the driver side, swap the MK1 fuel filter bracket to the corrado metering head. If I was you I'd bin all the corrado managment and go with kr stuff instead, but that choice is up to you. Oh and you want a KR inlet or a pair of ABF cams to pep up the 9a
One more thing if its not a 1.5+ already you need to chop off and reweld the driver side engine bracket from the chassis leg upside down and further back
