Your only realistic option is to get on the train at Napoli Centrale, even if you've bought a ticket starting at Afragola. It's the same price either way.
So the question really is whether there is enough time to get to Napoli Centrale from Molo Beverello if you arrive at 12:20 and need to be there by 13:22. The answer is yes, there is plenty of time.
You could take a taxi for the fixed fare of €13, a bus (#151) for €1,10, an express bus (Alibus) for €5, or the metro (line #1) for €1,10.
Thanks our tickets are definantly departing Afragola to Rome Termini at 13.40pm. Is there many taxis at the Port? I think we should be ok, but I don’t know how the distance tfe I’m the port to the station and was concerned about timing!
By taxi there is no fixed fare to Afragola so you'd have to pay the meter rate. The best case in the middle of the day is probably about 40 minutes (14 miles but through heavy city traffic).Edited: 15 May 2018, 21:35