I did this once. Be very careful, clean all the undercoating out of the square recess. The size I believe is metric, between 3/8 and 1/2 inch, to long ago for more info, equivalant. It must fit snugly or you will damage the recess and then you have real problems. Drain all the fuel before you start this. Spray heavily the threads with penitrating oil.
Yes the filter is available from
http://www.vp-autoparts.com and the part is 688593 Sarnfilter Filter petrol 121.00 krona
It is plastic and may be stuck on the metal tube that comes from the side of the tank, a pair of long nose pliers may do the trick to get it out.
It just slides on the tube and if really stuck you may break the tube.
Hope you luck out and the filter was cleaned ocassionaly.