"external program memory you must use the /PSEN pin for WO"

WO ??? no, for read.

There are a multitude of '51 derivatives with internal program memory:
80C51.. mask
83C51.. OTP
87C51.. EEPROM
89C51.. Flash

all of the above can be found in versions that otherwise match your 80C31.

Do remember to change the level on !EA.

Have fun,

Erik

Have fun,

Erik