Thinking about your situation I would get hold of an old Barchetta or HGT engine and re-build it while you still have the original lump in your car. There is a lot you can do to improve the stock lump (depending on how much you are willing to spend), lightweight flywheel, performance injectors, fast CAM - if you are feeling really enthusiastic Novitec do a turbo conversion for the Barchetta engine..................
I have done a few engine swaps in the past but only because the stock lumps were rubbish (which is not the case with the b). I had a 1972 Vauxhall Viva 1.3 and shoe horned a Ford RS2000 2.1 Pinto engine with dual twin webbers, also dropped a re-bored Porsche 914 engine into a VW beetle (OK, actually the same block but that was where the similarities ended)......
My advice, seek out a decent FIAT VVT engine, re-build it outside of your b and drop it on over a weekend once its finished.