Modifications on SAP Standard script data not displayed

Hi,

   I am facing problem on  SAP STANDARD SCRIPT .My requirement is  We can do some changes in in Purchase Order form   " MEDRUCK  "

   In standard the currency is not printing in terms of word so my requirement is i have to show the currency in words.

i am fallowing these steps:

1.Copying the MEDRUCK   to     zpo_MEDRUCK

2. After changing some changes in  ZPO_MEDRUCK

3. Through  NACE transaction i assigned to  Required program

These are changes in my form

In SE38 i am writing this code

DATA:LV_WAERS TYPE WAERS,
      LS_WORDS TYPE SPELL.
FORM F_AMOUNT_WORDS TABLES INPUT STRUCTURE ITCSY
                           OUTPUT STRUCTURE ITCSY  .

   DATA:LV_WAERS TYPE WAERS.

   READ TABLE INPUT WITH KEY NAME = 'EKKO-WAERS'.
   IF SY-SUBRC = 0.
     LV_WAERS = INPUT-VALUE.
   ENDIF.

   READ TABLE INPUT WITH KEY NAME = 'KOMK-FKWRT'.
   IF SY-SUBRC = 0.
     CALL FUNCTION 'SPELL_AMOUNT'
       EXPORTING
         AMOUNT    = INPUT-VALUE
         CURRENCY  = LV_WAERS
*       FILLER    = ' '
*       LANGUAGE  = SY-LANGU
       IMPORTING
         IN_WORDS  = LS_WORDS
       EXCEPTIONS
         NOT_FOUND = 1
         TOO_LARGE = 2
         OTHERS    = 3.
     READ TABLE OUTPUT WITH KEY NAME = 'LV_WORDS'.
     IF SY-SUBRC = 0.
       OUTPUT-VALUE = LS_WORDS-WORD.
       MODIFY OUTPUT INDEX SY-TABIX.
     ENDIF.
   ENDIF.
*
ENDFORM.

--->after assigning the modified form to Standard program my logic is not trigger only Converting currency to word format ..remaining changes will be updated