First, go to the plus logo and then get the Fuel Tank

After done, if you don't have, get the XML editor, you can activate it in the mods tab.

After that go and search
"massScale"
and "dragScale"
Turn both to 0.

Now go to the "FuelTank" in the XML.
And change fuel and capacity to:
1E+24 for infinite fuel.
And then it should look like this.

Before: