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IC choice
You want to pick a digital rheostat that will cover the range of 1k ('Back' button) to 20k Ohm ('Record' button). There are many such products in general, but if you want one for 50k Ohm in a DIP package you are basically stuck with Microchip, which are coincidentally the cheapest.
If you are fine with soldering SMD you get more options from Maxim and Analog Devices, as well as rheostat configurations (MCP4xx2) from MCP.
This is what I selected. If you run both channels in parallel you get about 100 Ohm per step, although it is not required. Works fine in single channel too. Two-channel devices have the advantage of separate SDI/SDO pins and a shutdown (SHDN) pin. MCP4251-503, MCP4261-503
MCP42050 might work if you can deal with high signal levels (VIH>0.7VDD) and slightly different command structure (see datasheet)
Fine as well, but Microchip ones come in a DIP8 package which means no shutdown pin and multiplexed SDI/SDO which makes it a bit less convenient. ~200 Ohm per step. MCP4151-503, MCP4161-503
MCP41050 not recommended due to high wiper resistance of 125 Ohm
Not recommended. A few products in SMD packages from Maxim and AD. With 20-30% tolerances you might be unlucky and get one which doesn't quite reach the required 19.5k Ohm value.
Same as above.
Prices are within a couple cents so not much reason to bother with 7-bit. For single-channel 50k pots you want 8 bits of resolution.
Sure, why not, but you are stuck with SMD parts. Needs changes to the script.
Knock yourself out, should be easy to adapt the script to work with it.