CV extension board needs +/-12V to do its job, so the main board has to be powered by 12V AC adapter. Because of this, the following changes need to be made:

• 7805 voltage regulator has to be mounted on a decent heat sink, or replaced with switching mode equivalent OKI-78SR-5/1.5-W36H-C (Digikey part# 811-2692-ND, Mouser 580-OKI78SR51.5W36HC)
• C3 should be changed to 220uF/25V. With this specification the capacitor is too high to fit between main board and 1/4″ jack connector on the CV board, so it has to be mounted on the top side of the pcb, see the picture below.
• MIDI IN/OUT LEDs and their current limiting resistors on the sides of MIDI connectors should not be mounted since  MCU ports that drive them are used to interface with CV board, so their flashes will be unrelated to MIDI activity.

I recommend to use OKI-78SR-5/1.5-W36H-C because even with a heat sink the 7805 can get pretty hot, especially if your LCD is one of those current hungry ones. With switching mode regulator the heat is not an issue at all and it’s easy to mount, see here: