It could be repaired with pewter without removing the tank providing you have a little space to work, all you need is a drop. It is often mixed with lead and the fusion point is very low, something like 180 degres Celsius. That means That you don't need a flame to melt it. A hot piece of metal that you hold with vise grip for instance would work.
I dont think that silicone whatever quality it is will resist gazoline very long.
There are shops specialised in fuel tank repair, these same shop also repair rads.
But realy the first thing to do is to take it back to the radio guys...Let's see what they propose...