Suspect it'd be easier to have a youth stock and a longer big kid's stock. However, the 1300 is just a slightly modified 1200 so anything for one fits the other. ATI makes an adjustable called a Shotforce that comes with a recoil pad. Runs a bit less($57 vs $69) at Cheaperthandirt than direct from ATI. Length of Pull: 12 1/8" - 15".
Not sure if it'll fit a 20 ga. though.