there are many 020 boxes that will fit, but not all have decent ratios to suit. a 16v box is close but it has a shorter 5th resulting in higher revs, and so higher mpg on motorways + theoretically a lower top speed
16v box requires a 16v clutch friction disc as the input shaft is bigger, as would the box out of a MK3 2.0 8v GTI. The MK3 GTI box has slightly more relaxed ratios as well, might not suit your driving style.
Other thing to worry about is boxes from 1.5-1.6 and MK1 GTI boxes have 90mm driveshaft flanges, which wont fit your driveshafts. Later Scirocco 1.8 boxes should be pretty close to yours in terms of ratios, depends on the year/code but they have 100mm shafts, as do later MK1 1.8 Cabbies.
