Today the fuel started to leak from the tank overflow
It’s a ZZR1400 on a 2007 plate
Do you think it’s the pipe inside the tank that’s rotted or split allowing fuel into the overflow tube
That sounds like it could be the problem, if your lucky you may be able to cut out the bad bit and fit some fuel-resistant pipe.
I don't know how much room you will have on a zzr 14 though, if the fuel pump is in the tank you may be able to remove that for more access.