that would be kinda wierd, only being able to turn at 90* - how would you actually hit somebody, unless you gun could move around on the bike or something.
Even more awesome... would be a 3D maze, where the bikes can go in any direction, up down left right. Ever heard of Snok? Look it up here:
http://www.tihlde.org/~torbjorv/snok/
That's the closest thing i could find to the idea.