i got this from another forum, and as far as i know its quite easy to all of these mods
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Right, here are a few effective things you can do to a Mk2 1.6 carb engine to get it roaring.....(in order of how I'd do it)
K+N panel filter in standard airbox, take off crappy foam pipe that leads into wing
1.8 8V downpipe (horse shoe clip type) + 1.8 8V gti exhaust system (or go one better and fit full stainless system for an 8V) The existing 1.6 manifold mates perfectly with new downpipe, new downpipe allows for 1.8 exhaust.
Swap camshaft with one from a 1.8 8V gti (I think 1.6's after '87 had hydraulic heads, therefore get camshaft from a hydraulic headded 8V gti, ones before have mechanical heads, so get cam from a mechanical head gti -Check before though!!
Weber 32/34 dmtl replacement carb (?190 new from GSF) - then get the whole lot rolling roaded and weber carb re-jetted (thats the beauty of them)
A 1.8 block or even 2.0l from Audi 80's etc can be bolted on to the bottom just as gti owners do, the 1.6 engine is virtually identical, except for the 1.6 block and the carburettor instead of injection system.......
hope this helps