They exist. http://www.theautomoto.com/
They used to cost over $4000, but I see the price has come down. I believe they use a pivoting system designed by Honda for their Gyro models several years back. I have a hard time seeing a 150cc GY6 powering something so big and heavy.
2002 Vulcan 750 (now being slowly reassembled) 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
2012 Zuma 125, 1980 Puch moped