There are quite a few different ones available. Some will fit in the original openings, others such as the small Chinese rectangles require a little filing of the holes and fabricating an adapter. I have used the Chinese ones from Ebay and made adapter discs.
Things to be aware of:
Some Hz gages require 12V DC power to operate, you don't want that. Find ones that are direct read and operate off 120V AC, remove the Frequency converter and connect the AC in and signal out wires together so you are now feeding 120VAC to the gage and use the original gage wires. Get a 250-300VAC volt meter and wire directly in place of the volt gage.
On the load / AMP meters you need to use a CT coil for the signal, If you get split coils you can snap them around the main AC wires near the breaker without disconnecting the wires to feed through the CT coil.
There was a member here making drop in gages using a nice 3D printed plastic mounting cup, but I have heard from others that they are no longer available.
Member Chb32NJ is currently working on adding digital gages to a couple of his machines, maybe he will see this and chime in, or maybe shoot him a PM and see what he is using.