Anyone know or tried an auto parts store oil cooler that will fit a 400ex pretty easy?
Also, anyone know of where to get an aluminum air box pretty reasonable?

 

Try looking at alamomotorsports.com under the permacool oil coolers. That will show you quite a few styles that are out there. I've never worked with one on a 400EX, but I'm sure you could make it work pretty easily. As for the airbox yours is already a clamp on filter setup, so you should try drilling a bunch of holes in the sides to improve the flow. The airboxes seem like a ripoff in my opinion.