Why on earth would you mention that you swapped out the pump?
Anyway...You could just put the original supplied pump back in, try calling them up again in a few days, chances are you will be speaking to a different person....don't mention the (irrelevant) swapping of the pump, and things will go fine with getting it replaced under warranty.
If you still find yourself stuck you should be able to find a local shop that can properly purge out the tank and weld it up for you.
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