The exposed bike and rider are going to create considerable drag. To
get to reasonable cruise speeds - I'm guessing 100-120 mph or so, to be
enough faster than a bike to make it worthwhile - it's going to take a
bigger engine than you think.

It's going to be difficult to adequately belt the pilot down (in?) if
he's sitting on a bike.

No IFR, of course, no space for a panel

Dealing with 100+ mph wind blast for an extended period is going to be
taxing.

No reason you can't have a conventional side stick for aileron and
elevator. Yes, you have enough degrees of freedom in handlebars, 2
twist grips and 2 grip levers. But it would mean a lot of training to
unlearn regular aircraft controls.

It comes down to defining your mission. If your goal is to ride your
motorcycle through the air, you may have a reasonable answer. If it's
to fly somewhere with your motorcycle, I'd lean toward a more
conventional airplane that can hoist or ramp a motorcycle inside, and
unload at the other end.