It would cost £1000's to do it, you could attempt a 1.4 Tfsi installed again costing thousands. I Just had a rebore, skimmed head, Skimmed block, polished crank, oversized pistons, rings, new valves and guides all on my Mk1 Golf and it cost close to a £1000. Lighten a crank, high lift cam, port etc and it would cost more than a 1.4 Tfsi install.
The problem with bolt on goodies is that unless you do many other parts to the engine its all compromised, you will always get a bottle neck somewhere and then the claimed figures are never achieved. You will always get people claiming they have the fastest most powerful A2 in the UK (On other forums), it does make me laugh.
If you want a faster car thats really cheap and you don't use the car for commuting, lose weight. You will gain a better performing car 0-60 and on track days from stripping it of weight, the rear seats alone weigh 6 stone for the 2!! Sound deadening, carpets, seats, air con, head lining, pretty much everything, even the 1.2 TDI windows would help, make sure you haven't got open sky system either as its very heavy and not good for the centre of gravity. A 911 964 Porsche in 1989 had 3.6mm glass instead of 4mm thick to save weight between the RS and standard model, it also removed the door cards and sounds deadening etc. It only came with 10BHP more but was clearly faster...
Example The Series 1 Elise was able to accelerate 0-60 mph (97 km/h) in 5.8 seconds despite its relatively low power output of 118 bhp.
I'm surprised someone hasn't done it already. You have a very stiff chassis, and reasonably low weight to start with