The x0xi0 should work , its just hardware mods , yeah?
yes, it will work fine with the standard x0x pcb.
The custom mainboard is different though, much tighter, it's all the way on the side. With the current layout the adapter pcb would have to be located in a different spot and wire/cable used to connect to the existing footprint.
Regarding the crystal, definitely put it on the new board right next to the micro.