This is an oldy but a goody!
If you are not concerned about damaging any tank lining material (I doubt CT90's would have any?) COUNT OUT as many old small nuts or bolts etc that you can get your hand on and then dump them inside the tank with a cup of fresh fuel and close the cap.
GENTLY swish the whole lot around for as long as you can then tip out the muck that results. You dont have to madly rattle the whole lot around in fact doing so may even damagfe the tank depending on the size of whatever you have put inside.
Keep repeating this until the fuel comes out clean and the inside looks to your liking then COUNT the nuts etc again so none are left to rattle around inside! It is handy to extract the said foreign objects with one of those fancy three pronged part picker uperers that many auto and engineering places sell.
Job done !
AND THEN
Line the tank with a good quality fuel tank liner kit, but that is another thread.
Cheers
Dave Mac