Good news: this board has conventional capacitors, so no leaking here.
The battery must be replaced, so I've ordered one (3V).
I've removed the roms from the sockets and left the board in hot water with a shot of ammonia.
After that, I used a teeth brush to clean up the board, then used a PCB cleaner to finish it.
Then I used contact spray on the sockets, placed the chips back in place and the main board is ready to go, except for the battery I have to wait for.