First of all it smells like fuel, not motor oil? Because your chain case drains from this same cover underneath. Plug for fuel tank faces the rear plug for chain case to the front.
I doubt there is anyway to repair the fuel tank, even if you get it out (which requires engine removal) it would have to be replaced.
Some of these tanks failed it very rough application such as concrete demo work, running solid tires etc. If that sounds like you machine then tank failure is more likely.
However I would first make certain it does not have a bad fuel filler hose or air vent hose (both connect to the fuel filler), or a leaking hose going to or from the engine, or is leaking around the fuel level sending unit, and finally that it is not simply leaking out the drain plug underneath the cover you mention.
If its oil it is likely the chaincase plug, there is no pressure in the chain case just gravity pushing the oil out, so it's not likely to get to be a bad leak. Chain case lever can be check at a plug in the lower front center of the machine just above the chain or tie down loop for transporting the loader
Are you adding any oil to your hydraulics or chain case on a regular basis?
We'll all pray fro a simple fix.