Just out of curiosity, why do you wish to change the alternator?
You can date the engine precisely from the Code number stamped into the blower housing ( engine cowling) if it was made in 1980 the number will start 80 followed by the day and the month, say 2408. Any digits after that refer to the shift/ production line on which it was made. Assuming that it’s the original engine the tractor was probably made within six months of it.
Westwoods of that era usually has a sticker which often included the year of manufacture. Sometimes the sticker was on the inner engine side of the steering wheel console but could also be on the inner chassis side plate by the cut out for the spare fuel can, or if the machine is so equipped, under the seat in the opening for the power take off pulley for the power grass sweeper.