If it is an OBD1 PR3 then it is basically the same as a P28 or OBD1 P30. Cut J1 and you are back on the stock maps.
However, if it an OBD0 PR3 the old ROM was removed, a socket was soldered in, and a new chip was installed.
If this is the case you will need to get a new chip with the stock PR3 maps to return your ECU to stock.
-kenji