Hi Jules,
if you are completely sure that the motherboard is down and its not just a bad connection to the screen or anything like that you can try to fix it by heating it:
This thread for instance suggests to heat up the mainboard (only mainboard without the metal cover on it or the frame) at 200°C for 10-15 min (there are multiple threads about heating up the FP3 mainboard on this forum). This is obviously at your own risk but can work.
If you are not interested to try that, I would like to purchase the broken mainboard if possible. In that case just give me a direct message.
All the best,
Clemens