A mate of mine had a diesel box on his mk1 (this was ten years ago) that had been on there for a few years. 1.5 springs to mind but that can't be right, they only made about 3bhp and 1lb/ft so would break in no time Im sure. Try looking at a 1.6 GTD box, those engines get wound up over 200bhp and the box still takes it, so Im sure it could handle GTI output. I think all mk2 boxes suffer from the same self destructing diff and crunchy second gear, so it may be best to just get yours refurbed (or buy another for £50 and refurb that) Should be good for another 100k + miles then. Assuming that's what's wrong with yours that is!