I recall the Dodge Tomahawk from some years back..that was basically a Viper engine in a motorcycle:
Technically it wasn't street legal. Might not have even been technically a motorcycle, considering it had 4 tires!
Boss Hoss has done some crazy builds as well. From 2000-2008 they had one called "Stud Hoss" that used a 502ci big block Chev engine.
Largest production motorcycle tho, has to be the Gunbus 410:
410ci (6728cc) V-twin
~350HP, ~525 ft/lbs torque
No matter what, there's always someone more suicidal than the next man.