I tried it. And failed. After 4 hours of skinned knuckles from moving driveshafts and manipulating bits of the subframe and drivetrain around I had to admit defeat. The problem is although the head end of the manifold is an exact match on the 16v and Willy, the downpipes arent interchangeable and the downpipe will not meet the cat section with the correct angle and length, in fact it will barely even make it past the subframe. I dont know why this should be...theoretically it should be a bolt on, but it just isnt. Plus the cat-downpipe fixing flange is at the wrong angle as well, although you could cut the flange then turn it 90 degrees and re-weld it if you wanted to make it fit.
According to Nick Hill, the 16v manifold works better on the 1.8 and the 2.0 lumps anyway. Apart from anything else its bigger (although I cant imagine the airflow is as fast, what with all the twists and turns on the 16v one).