If you'd prefer not to hunt down all of the hardware yourself you can buy these kits with all of the necessary hardware here: https://bit-district.com/midway.html
What is it:
Midway Y-Unit and T-Unit Arcade hardware (used for Mortal Kombat, Mortal Kombat II, NBA Jam, Smash TV and many other games) has 2 separate PCBs, one main board and a separate audio board, in addition to a remotely mounted Volume Pot.
These parts are designed to help you mount the audio board on top of the main board as a more easily managed PCB stack in addition to integrating the Volume Pot, making the PCB easier to manage.
What other parts are needed
The volume pot holders are designed for volume pots that are 18mm or less in diameter and 10mm or less deep.
You'll also want M3 truss head bolts 24mm in length or longer and associated nuts.