The current motherboard is an Arduino Mega with a custom PCB. The PCB was designed in KiCAD. KiCAD files - including the latest schematic and PCB layout - is in the Sixi 2 Github repository.
We are still trying to build a process in the shop so that the STL files and BOM are always up to date with our current work. In the mean time, have a read-only look at our Sixi 2 Fusion360 project. You may be able to extract the BOM and STL files from there.
As opportunity arises we will be making videos showing assembly of each sub-component. If you are equipped with the parts and tools you should be able to follow the video playlist or the pictoral guide and build it yourself.
How to apply
Don't just send a generic "I think I'd be good" message. Make your case.