Since Zenith6S321 seems to be MIA at the moment, therefore can't provide the mod kit, the best approach would be to download the EPROM data file from the earlier posted threads. Then send it to Matt's Basement Arcade to have a EPROM burned.
Website;
https://mattsbasementarcade.com/
On ebay:
https://www.ebay.com/itm/23267907567...gAAOSw3zNak5mR
EDIT: You will need a miniature DPDT toggle switch, for a thick (plastic) panel, like this one.
EDIT:
https://www.mouser.com/ProductDetail...YPLoz4kA%3D%3D
You might find it cheaper elsewhere.
The kit included a water slide decal to label the switch, you you have to come up with your own label.
A CDROM with a lot of info was included containing the schematics, operator manual, and a Maintenance (Service) manual.
Most of that info has been uploaded to the BAMA site now.
https://bama.edebris.com/manuals/sencore/va62/