If this is something you are keeping and want it tip top shape, I would see if you could have it media blasted inside. Next just nuts and bolts as you did with Diesel. There are several motorcycle gas tank fixes out there. One uses and acid then neutralizes. Let that dry then pour in Kreem sealant. They work good if you do the prep work.
If the tank is in bad shape, one could weld on a new bottom piece of steel.