i looked into it when i blew up my 1.2 lump, as far as i know the stock intake is the weak point, so for higher rpm and much more power you need to replace that so your looking at using ITB's or be oldskool and save money and use motorbike carbs!! (my mate has a 1600gte nova lump running them and the noise is mental!
SuperSprint make a performance tubular manifold (not sure if its a 4-2-1 or a 4-1 tho! cant remember off the top of my head) K-Tec also make a tubular manifold for it.
Supersprint make a decat for them as well as a centre section and back box.
cams are availible from various companys.
personally if i was going to do it id start with getting a 1600 16v conversion kit from matt black £900 odd + VAT from what i remember, have it fully rebuilt, and the bottom end lightned and ballanced to help it rev have eather the standard cams reprofiled or go for some aftermarket ones, tubular manifold, stainless exhaust and decat as well as running standalone management so in future if you wanted you could go the ITB route.
at the end of the day i was ready to go ahead with it then test drove a 172 and thought f**k it, put another 1200 engine in my dynamique and bought a 172!!