the zxr250's didnt come with an inline filter, only one in the tank before your fuel tap.
these are usually pretty good just unbolt the fuel tap (two bolts on bottom of tank) and give it a good clean. if not contact a guy "YUKI" from southern cross sports, he can get these in if your ones buggered.
if you want to install an inline filter make shore you get a small thin one
, as theres not alot of room and you dont want to pinch the fuel line.
as for rust treatment i have used the septone stuff from supercheap auto and has done pretty well, empty the tank of petrol remove fuel tap (good time to give it a clean) remove the fuel filler cap (so you dont damage seals) plug off holes from fuel tap and filler with rags and tape, then put some rust converter in and give it a swish around for about 30-1hr letting it sit every now and then on different parts of the tank (i.e tip on left side for 5 mins, give it a shake, tip on right side for 5 mins, give a shake etc etc)
if you have alot of rust and flakey bits, get a few small nuts and tie a bit of string through them and chuck in them and give it a shake to loosen it up
then to clean the crud out use methloated spirits till it comes out as it goes in,,,,, then place everything back on tank and put back on bike quite easy and can be done on a rainy day with ease
and all it will cost ya is about $40, $20 for 1 litre of rust converter and about the same for the metho, get about 5 litres to ensure a good clean of your tank.
if you dont wanna use metho i have used petrol as well to clean it out, i ran out of metho
haha
cheers hope this helps
lolz.