Yep Warren, there have been a few instances of this design to make it into production, but as far as I know these were and are liquid-cooled single-cylinder dirtbikes, the short-lived Cannondale E440 of a few years back, and the latest model(since 2010) Yamaha YZ450F.
A few years ago I read an answer to this very question of yours as to why this didn't or hasn't become the norm in motorcycle design in an issue of Cycle World magazine. Kevin Cameron, CW's Technical Editor stated that the reason is that an engine gets hottest around the area of the exhaust valves, and thus because of that and the need to keep that portion of an engine cool, and especially so on an air-cooled engine, that always necessitated motorcycle design to have its exhaust forward in order to receive the most amount of cooling air possible.
