Here's the equivalent CSC-1 assembler code, which is a complete program ready to run in CASM:

MAIN:
;  x and y are already declared and set at the end
;  call the subroutine and leave the result in A register
             LOD   X
             STD   MONUS_PARM1
             LOD   Y
             STD   MONUS_PARM2
             CAL   MONUS
             LOD   MONUS_RETVAL           ; leave it in Acc
             STD   4095             ; print answer
             HLT

; variables for main
X:           NUM   8
Y:           NUM   5
ANSWER:      NUM   0

MONUS:
MONUS_IF1:   LOD   MONUS_PARM1
             SUB   MONUS_PARM2
             JN    MONUS_ELSE1
             STD   MONUS_RETVAL
             JMP   MONUS_ENDIF1
MONUS_ELSE1: LDI    0
             STD   MONUS_RETVAL
MONUS_ENDIF1:NOP
             RET

; variables for monus
MONUS_PARM1: NUM   0
MONUS_PARM2: NUM   0
MONUS_RETVAL:NUM   0