I have a mid 1990s electric wall heater that is wired to a King, 22 amp, in-line voltage, single pole thermostat. Is it possible to upgrade this to a digital thermostat - and if so, what brand model would you recommend (if one exists)?

Thanks in advance.

Sure lots of options.
For example a Google search shows this one

Thermostats - Line Voltage Thermostat - Honeywell TL8230A1003 - 40 to 86 (F) / 240 V - Digital Programmable