Does the front derailleur limit chain ring size?
As Criggie says front derailleurs have a maximum chainring difference and min and max tooth counts. the more modern a derailleur and shifter is the more important this is. With an old friction shifter you can probably push this quite a lot.

The bigger issue is the 'bolt circle diameter' on the chainrings and crank. It's probably wide enough to preclude a ring much smaller than 40 teeth, if you want a smaller ring you'll need to find an alternative crank.