If I was going to make a fazer 'cafe racer', which I wouldn't, because although I love my incredibly practical fazer, I don't think the main elements are particularly pretty, like the engine, tank, frame or swing arm. I wouldn't say either were show pieces. Although I'm really looking forward to being proven wrong and eating my words.
If it was me, I would cut out the existing filler and weld in a flat plate, and put a nice shiny cap and filler neck on the front right hand side. You can get some really nice filler caps and if you put it on the right you can brim the tank with the bike on the side stand.
I'm thinking about making a custom bike, so have looked at my fazer (and google images 'fzs600 cafe racer'), but don't think its the best starting point. I reckon I'll get an xvs and turn it into a lowrider, fat front tyre, stretch out the riding position and fit some bullhorn wide bars. Each to their own huh! ;-)