For normally aspirated power on your KV Mellis is right so expect max 180 bhp with a heavily modified head, will probably need bigger injectors as you'll be maxing out the duty cycle of the standard ones if they go that far which i doubt, you'll need exhaust improvements and radiator so this is not going to be cheap.
There is a normally aspirated 90 7A sport around which visits ADI most years that pushes out 260 bhp, that's not a typo, with a massively reworked block head ecu etc etc.It's quick but seriously drinks fuel.
The best bet i'm afraid is an ABY or an AAN 20Vt that's going to cost you a minimum of £3k just to buy the standard engine ecu, injectors ancillaries then there's a custom exhaust on top so your looking at best part of £4k this route.These parts are getting expensive now.
To get 350bhp from the ABY or AAN it needs to be RS2'd. You'll need injectors a chipset RS2 K24 7200 turbo and exh manifold that's at least £1.5k from prices around 2 years ago now second hand and will look stock or go with the Volvo K24 7400 turbo circa £400 + used and porsche 944 K26? hot side £££ second hand with a new special made Cartec EVO 2 RS Sport exh manifold thats another £500 new on a group buy and suitable Bosch injectors and the route i'm going down with the S6.So there's at least another £1600 then custom downpipes exhaust and a remap.££££
Having said all that there is a guy who did a 10v turbo conversion on his B3 coupe quattro, just need to find the thread.But it's your worst nightmare.
10V Turbo B3 some of you will recognise it.
