Not wanting to piss on your fire but unless you spend thousands on the engine and forced induction you wont get much past 150 bhp.
To hit 150 bhp from a measly 115 bhp you will need a cam, 4 branch, porting and flowing, ecu/map/custom ecu, balananced and lightened fly/crank etc etc etc, And lots of free time and friends.
If you have time and feel like a laugh then go for it, someone on here did a cross flow head 8v and got cool results, have a scan through the build threads to find it, iirc he had friends time and money
If this is not you then these are the options.
If your skint, live with it and spend you time chasing the rust.
Sell it and buy a VR or 16v
Or if your mad, generally deficient of sense or need to get away from the wife then consider an engine change.
Cheapest and easiest is to dump another Mk3 engine in, maybe £500 parts + labour
Next move is to look at a 20v turbo unit from other vag machines, maybe £2k parts + labour
This has been covered a bazillion times, use the search function to avoid further sarcasm
