Odroid C2 arrived. The XU4 is based on the Arm V7 core, and so is eight cores but only 32-bit, by the way, the C2 is quad core, but Arm V8 cores and thus 64-bit.
Physically fits the case, but you lose the 3.5mm audio out, as the C2 doesn’t have one. 40-way connector is also in the same place and the hi-fi berry card mounts ok. But it will not boot the standard Zynthian build from the micro-sd card. You just get a white screen.
I am going to download the C2 version of Debian (https://forum.odroid.com/viewtopic.php?t=18771) and see what I can do with it. The C2 is supported by libreelec and other common RPi distributions, so I am hoping it won’t be too difficult to get it working.
If I make any progress, I will let you know.