If you can get the remaining nuts off, you should be able to remove the sender. I replaced mine a few years ago but it was different in that I needed to unscrew the whole item. In my case it was necessary to soak in PlusGas and leave it a week, when it came off fairly easily. I would do the same for your remaining screws and the broken stubs. It might then be possible to remove the stubs with a mole clamp.
Second thoughts on this. If you cut or shear the heads off the bolts, the fitting should lift off and the remainder of the bolts can be filed/ground down flush with the tank. If the fitting is then rotated, new holes can be drilled and tapped between the existing ones.