FJC --> the custom ROM in picture is PAL 7.7. I read the chip with my eprom programmer and the md5 hash is exactly as the ones linked here:
http://atariage.com/forums/topic/108472-xf551-oses/?do=findComment&comment=1316075, and here:
http://atariage.com/forums/topic/108472-xf551-oses/?do=findComment&comment=1376171
The drive works with Hyper XF-A ROM too (better).
Mathy --> I hope you slept well
Thank you for all replies and advice. Due to the fact I own another xf551, I think I'd rather sell the drive pictured here as it is, so someone with better electronic skills than me can repair it.
Cheers,
µicr0Bee