WOW! The fitting you hold in your hand in the pic sorta' tells all. YOU GOT RUST! And rust can easily destroy anything like your injection pump if it gets past your filter(s).
If it were me I'd pull the tank and burn it out, possibly by using gasoline, CAREFULLY! or diesel fuel or a combo of both until there is no evidence of anything but clean bare metal in your tank.
If you can't get it damnn near perfectly clean yourself, including trying the sharp rocks trick, it maybe time to hire a pro who has the tools/experience to do it right. Think about it - without correct clean fuel you have no tractor to use. What is it worth to you to have a reliable tractor?
I had an '87 Izuzu Pup truck which I owned since new. At one point it started running weird. Turned out the fuel tank was rusted. I took it to a guy who had the tank lined and guaranteed it would NEVER rust again. Way less $$$ than a new tank. Never had any more problems with it after that. Rust, combined with biological agents can drive one nutz!