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 subcomponent. Thus if you are equipped with the parts and tools you should be able to follow the video playlist and build it yourself.
Note that this project is still in development, and many of the things we build turn out to be dead ends. You may find yourself pouring a lot of cash and time into dead ends that get cut from the final version.
If you'd prefer instead to fund our effort while you sit back and watch, join my patreon. Of the two it's the more economical solution.