Hi jespera, congratulations on scoring a decent Cube, they are few and far between on Craiglist these days.
One of the key functions you will require on the motherboard is 5V Standby Power. This is necessary for the touch sensor functionality to operate, which is a critical part of the build if you want to keep the touch function on top of the Cube shell.
Different solutions have been used with PicoPSU's, but by far (as kiwi mentioned above) the DQ77KB has a featureset that is nearly perfect for Cube builds. My i7 Cube scores around 13000 on Geekbench 2, and runs very cool in normal operation.
Pioneering a new Motherboard in a project such as this is bold, but risky and potentially more expensive if it doesn't work properly. You only get one chance to cut the I/O shield hole in the bottom of the Cube shell, so it has to be done precisely for the correct motherboard on the first try.
Good Luck!
Ersterhernd