They all fit without mods but there are differences.
98/99 18 litre
00/01 20 litre
02/03 22 litre, but different shape to the others
I have just found this out because I won a Bagster tank cover on Ebay then it dawned on me that 02/03 covers were listed separately to 98/01 covers.
Not sure what to do yet, whether to sell it on or upgrade to the bigger different shape tank so I can still use it?
Problem is finding a tank in good condition.
Lo and behold, it's literally just been handed to me at work as I'm writing this.