Replace the lithium button cell (within the camera) that holds the ISO and other set data when the battery is removed (or batteries, plural, in the case of the early version of the power drive booster, if used). Additionally, inspect all circuits for functionality/corrosion, especially the display driver (governs viewfinder and external display information).
This [battery replacement] is a service normally carried out after 5-10 years at Canon, or at the most, when the ISO and custom settings are lost. Additional problems can cause this; Canon has a tool that plugs into the hotshoe to read-off the camera's system. The EOS 1 really should be inspected/serviced; it is now more than 28 years old (first popped up in 1989).