you would need to see specialist like for all cars that require high tune
waynne already mentioned awesome along with the link i gave you to the engine part of their website
for most cars general tuning companies like
http://torquecars.potn.com isnt going to have masses of stuff available for all cars as most "boyracers" (using the term as the ones who race up and down the high streets bouncing off the limiter) just get the induction kit exhaust and chip done. its only the enthusiasts (ones that will do the car up for themselves and try and make it better all round to live with ,reliability ,and for fun) that will look further than that ie internals
ive had the bora just over a month now and the only thing i have been unable to find for the engine is
1. cams (found pulley set but no cams )
2. crank
3. clutch but i think its the same as the 1 for the 5 speed 'box if so sorted
4. money to do the thing up with
found masses for it by looking at vag tuning groups such as awesome ans jsb
heads, rods found on a lot of pages turbo conversions on all of them (inc a remade K03 turbo housing with different internals to give 240bhp without rewrite and still looks standard. dammed if i can find it again tho)
re-bores to 1900cc
think theres proof that its a tunable engine when 350bhp is common enough for them
dont know if youve seen it there a vid floating about on the net with a mk3 with the 1.8t engine kicking a supras a**e on the quarter mile beats it by over a second