In my NTBH opinion, the ULTIMATE 'Frankenbike' (assuming by that, you mean: sport-bike engine in a street-bike/cruiser-frame) would be the ultra-rare Kawasaki Eliminator's...mainly the 600, 900 and 1000's.
It's amazing to me that all three versions had such a short production-run, given their rock-solid performance & reliability.
I had an 87' 1000 Eliminator briefly...my only reason for getting rid of her, was mainly the fuel-economy...no matter how 'light' a wrist I drove her with, she never once gave me 100 miles to a full-tank of gas...
...truth be told, the 'Millennium-Falcon-esche' launches she was capable of, seldom made for great gas-mileage anyways...lol
Not easily 'programmed, categorized or referenced'.