Yeah, when I first saw a picture of one of these , I was insantly convinced that I had to build one too.
The easyest carb choice in my opinion would be one of the carbs off a 2500 triumph.
Half of 2500 is 1250, nearly perfect for a 1200. Plus a mate of mines dad has wrecked literally hundreads of them, so I can get a good working carb for $30NZD.
Im waiting to find a turbo with some life left in it for the right price(Im much too tight to splash out for a flash one.

re: those carb calcs.

one would be fine for an NA engine, but for forced induction you'll want something off an engine around double the size - hence why the 32/36s or a 2in SU works so well.

Yeah but, the more air it flows the richer the carb gets. It's all relative to air flow. the turbo cant pump air into the motor wiyhout it passing through the carb first-pulling in the required amont of fuel.
I would think that the needle would need a wee bit of modifiying to make it run a bit richer, but tit wouldnt need much Beacause the air fuel ratio of the carburator should be about right.