Within Farm Works - when creating the new crop enterprise it uses the map found on the Boundary Layer to assign to that crop. When creating multiple enterprises for same field, you are correct the map will always be the same - causing to edit the map on the Enterprise to display just that portion of the field.
If you no longer want to edit those maps, you can create a separate field for this 29 acre parcel. Get the map section removed from the other field and use this map to get just the 29 acre parcel to the new field name. Once the Boundary layer has been corrected - when creating the new crop enterprise for the new field it will only use that parcel map as well.
I have attached a link to an FAQ that may help with correcting the field boundaries or edited the crop enterprise maps is you chose not to create the new field.