I hate it when people think a cafe racer is just a set of bars and a seat. Nobody thinks I can turn an r1 into an adventure bike just by putting on some nobbies, so why think that about a vintage performance motorcycle and some shitty handmade parts.
If you are going to "build" a bike, then really improve it. Solve the problems the stock chassis has, make it really fun to ride, not just something to impress high school kids with in parking lots.
There is a little place called ritmo sereno in Japan that can sell you anything you want to make your bike look good in a weekend. But without the mechanical improvements you are just a no-go showboat. If you look at the bikes they build as showpieces - lot of big stick to back up their looks, case in point:
Read the write up and see how much went into making this bike.
Anyway, for a good starting point with any performance machine is to look at he race bikes of the same type. After all you are building a race bike for the street. Start with the Daytona winning 1976 r90s of Steve McLaughlin.