First I set up a perspective grid, something I've never actually done before. I will do so from now on; it's had a huge impact, especially when drawing buildings. The motorcycle is just blocked in at this point, but I realized too late that I was drawing it from the wrong angle.
I started going over the parts using a more detailed photo reference, fixing the angles and working on rendering.
Here's the finished underpainting for the character and the motorcycle. I was dumb and painted everything on one layer, so I duplicated that layer and erased the background from the top. This is with the background layer turned off.