The Ashida is probably the easiest to build, seeing as you can just buy the custom PCBs from 4layertech. G-Wii is second easiest, but doesn't lose by too much. Really comes down to which design you like the most
Those are Rev2 Boards, the G-Boy uses/used proprietary versions of the V1 Boards. You'd have to redesign the internals of the case to fit the new Boards and still design some for the triggers as those weren't sold in any way or form apart from the G-Boy.
Oh you said G-Boy......... my brain autocorrected to G-Wii....... Yeah making a G-Boy from the CAD files alone would be an undertaking, you'd have to design all new custom PCBs in order to use the new 4layertech boards with it.