Biological growth in the fuel is probably clogging the fuel flow out of thank, through fuel lines and fuel system.
Start by looking at the flow out of the tank to the fuel filter. Remove the fuel line and drain it into a clean container but through a paint strainer to catch debris.
There are fuel additives that help break it down and others help keep it clean. If it is in the injection pump and the injectors they may need to be professionally cleaned. I don’t know if the additives can help in that situation but since it does run perhaps you can just add clean fuel with the additives, bleed the system if needed and let it idle for some time to get the additive into the pump and injectors. Then it may need to sit for some hours to break the sludge down, then maybe you can run it harder (but stay close to the shop) and see if it clears up.
Good luck.
