The 1300 is fairly limited in accessories, although FN essentially makes the same gun with collapsable AR stock, etc. Unfortunately, they aren't offering those goodies for sale.
When I got my Defender, I put Hogue stock and foreend on it, a side sling swivel bracket (since having a sling swivel so close to the muzzle worried me), a CQB solutions sling (SOP Sling with ERB for CAR-15 with fixed stock), and a Tacstar side saddle. I may eventually fork some cash down for a Surefire foreend when I can do some night shooting (shotgun matches).
I have thought about finding a competent smith to file off a little of the receiver (maybe a 1/4 inch) to make it flat enough for a rail to be bolted on, so I can have the option of an OKO sight, but don't know if that would really be worth it.
-Zmeja