to actually bore out the engine machining costs are about 150 quid inc honing and prep. BUT, you would need to find pistons that would fit the bore size and im not sure youcould get very much larger in the bore. +90 though would be a fair bit, and not really worth it.
Basically, you wont get more than 120bhp out of your engine in NA form without spending a shed load of cash com custome parts/machining gas flowed heads. Then we would need to work on a cam profile that would work, then get springs hat fitted or made up, then get custom valves etc etc...it would go on and on and were talking the price diff to buy a faster car and pay the insurance.
Just wait a while, this board is going metal with engine swaps, but rarely do people understand just how hard it is to swap motors, and tunning of the REAL variety is not cheap if the engine isnt alreadya popular engine to tune. Custom parts cost alot and development is lengthy.