New and in need of help with engine conversion!

Hi all, I own a 1984 gti karmann and am looking to improve performance. After some consideration i am leaning towards getting a mk2 gti 16v donar engine and fitting it with twin 45 webber carbs. Can anyone give me an idea of any problems i may come across doing this and what the likely power output will be of a 16v with weber cars and a good set of cams would be. If anyone has any advise why i shouldnt go down this route please let me know! Cheers