Acetone will soften epoxy. If silicone then brake fluid. Of course, either will ruin the paint and likely the badge..
It will attack the plastic badge and wash the colour off it. I’d Stay away from anything that is a ketone or petroleum based.
You could go all nutty professor on it, wash the tank out with a tank cleaning solution and apply a blow lamp to the inside of the tank (remove the taps) the tank will get so hot that it will break the bond with the steel tank.
Down sides are:
You may not have removed all the fuel (I’ve done it twice and been lucky. You usually just get some fire out of the top)
If it’s a chrome tank then it might blue/yellow the chrome, these tanks aren’t as thick steel as exhaust pipes.
It will definitely destroy the paint and turn it to powder (which is why I do it)
And after all that it’ll be Sod’s law that the badges are still stubborn and stuck to the tank.
Final option, spend to accumulate. Buy some badges and smash the old ones off, then deal with the mess left behind.