I have found that the water tank has been fouled by bilge water that has seeped through the seal on the hatch. Can you still buy these seals or can you buy a new hatch.
Most twisters were fitted with the Henderson TCL3 oval cam-lock type tank cover/filler. These are still available from good chandlers though the cost may make your eyes water. You may also be able to get loose seals for them.
A good idea, if this is the type you have, would be to replace it mounted on a spacer so the hatch is up clear of any bilge water that is swilling around. Make sure you bed it well in silicone or other suitable sealant and preferably through bolt with SS machine screws. Use Duralac under the heads of these where they touch the aluminium to minimise corrosion. If you are really being a perfectionist you could get the frame anodised (I can't imagine why the makers don't) which should stop a lot of the corrosion or just paint it.