I’d go for a cargo carrier or trunk instead of a box. More space to put stuff in it.
The mirrors are not necessary, you should be able to see behind you just fine without them. You could put some blades in their places instead, for some points in badassery.
You might not be able to properly power an electric engine with only 1 solar panel. Even if you put solar panels on most of the vehicle, it might not be enough, especially not for longer trips. It would be acceptable as an emergency engine though, I guess.
You might want to put some seatbelts on your seat 
Put a window or something similar on the front wheel. Anything you can see through and stops movement through it, basically. You don’t want stuff to hit you directly if you happen to ram something in the way. Even something small can cause quite some damage on your character if the speed is high enough.
You could try a different version of this bike with either a welding station or a fridge attached to the point where the box currently is. Both of those options can hold a certain amount of items as well and have more functions, they do block your movement though and drain power when active. Still, a place to put your perishables, or a vehicle mounted welding tool to fix any damages might be extremely useful for you, especially if you’re going hybrid.
Again, a single solar panel won’t be enough to power a fridge nonstop - even 3-4 aren’t enough I believe, dependinig on the solar panel, but you don’t have to keep it running all the time, so…yea.
You might want to put frames or something similar on the sides of your seat. That way you can put a roof on all 9 tiles and your Seat would be considered to be completely roofed, negating any debuffs from being in the rain. Those can’t stack up quite high if you have to drive around in the rain for a while.
While speaking of rain, with an extra tank, a funnel and a faucet you could have your vehicle automatically catch rainwater. You still need to clean the water, but having some spare rainwater might be very useful.
Some of my ideas for your little motorcycle, Grand Sage. Most of them should be quite easy to do and would improve the overall utility of the vehicle. Of course you wouldn’t be able to do all of them, due to the small size of the vehicle, but that’s what I’m workign with.