fred99999au said exactly what i was going to suggest, drill head off, get tap out the way then way then work out what to do after that when you can get a good look underneath.
You could also possibly try and drill throught the center of the screw, slowly working your way up in drill bit size till you are just smaller than the ID of the thread, but you will have to work out how deep you can drill it going by the depth of the screw on the other side and mark your drill bit with some masking tape so you don't drill too far. Then you may be left with just the remains of the original screw threads you can try and dig/unscrew out. If it works out as planned the original insert thread will be left intact but you will still need to work out a way to resecure the insert in the plastic again. Worst case scenario is you cant reuse the 2 bolt type tap because of loose inserts, so instead use a Canam rubber bung style fitting & tap or a universal Malcolm Smith type threaded spigot that you insert through the inside of the tank. Both will need drilling outlet hole in plastic to about 1/2". So there is this solution available if things so bad with the original 2 inserts.
A thing like this is for plastic tanks and has a o-ring on inside. You thread it through the filler neck hole on a piece of wire. This particular one uses a 1/8 NPT universal tap that fits the thread on the ID, but you could make a fitting that uses a tap that has a female nut, the same set up as early Suzuki RL, PE RM alloy tanks.
