Maybe not an "H5 motherboard for Mega ST", but just a possibility to reconstruct a better PCB, someone motivated enough (not me for now) and with said Mega ST at hand (definitively not me) could do the same kind of work that I did with the Atari STe : using Kicad, capture the schematic, and reconstruct a PCB (no need to be precise to the mm at first). And do it 4 layers for better grounding, as a starter.
The service manual of the Mega ST even have the top and bottom layer of the PCB, even crappy because these are crappy scan, that's better than just pictures of the motherboards.
For the Mega STE, it's possible, but it start to be a lot of work, compared to the "simple" STe.
It does not require high skill in electronic (sure, the more the better), just a lot of free time and cares.
For better replica, one really needs an naked motherboard, for scanning a reference layout.