Unfortunately ( I Guess) nearly every fast bike is going to need to be built rather than purchased. Nearly every quick bike I've put together required some manner of kustom part or jury-rigged something or other. Even the manifold that came with my Gilardoni back in the day had to get scrapped because it wouldn't flow a 21mm.
Fastest 'no-work' high $$$ set-up:
OLD! Not Current! Polini cylinder 46.5mm piston, greatly resembles a Gilardoni.
Simonini exhaust or Homoet 8p
Mikuni TMS 24mm
Fastest High $$$ Lots of Work set-up (under 100cc's):
Eccentric Crank Pin on KX85 rod (41mm stroke)
Custom crank web (flywheel side) with 20mm cone
Weld up engine case, relocate cylinder studs
Cristofolini 90cc LC cylinder for Minarelli AM6 motor
KX85 Powervalve assembly, grafted to cylinder, slaved electronically
Parmakit Variable Ignition advance (for Vespa)
Kustom case induction reed block & Malossi MHR Manifold for Gilera Runner
Moto Tassarini V-Force reed block (for Suzuki RM85)
Fabrizi Racing Exhaust for Malaguti F12
Keihin D-Slide 30mm Carburetor
-There's probably some guy in the netherlands with this bike already, but if anybody stateside builds this up, I will totally give you a high five.