The game started but colours were wrong on some sprites and others had vertical bars on top of them (hard to see on the pictures). Furthermore sound was very low with a humming noise.
The part of the board handling sprites is in a corner with 2148 type RAMs (55ns!). 3 of them have been "beheaded", probably due to poor storage. I installed brand new ones @ A27, A28 and B27. This fixed the sprites issue:
Concerning sound, I replaced all electrochemical type capacitors but that didn't changed anything (but it can't harm, original ones were 29 years old!). Then I "listened" to the signal at the output of the pre-amp: it was clear. So I simply replaced the audio amp (MB3770) and sound was restored.