I REALY like this idea. the positioning of the figure makes perfect sense. I would add a footpad or two behind the figure to prevent the "superman" position effect of him just holding onto the handlebars. At the same time I would remove the footpeddals from the underside, they are pretty useless down there if the figure isn't going to be sitting upright.
I like all the greeblie details you used and the new gun on top looks sharp too.