I aint no mechanic,,
Could it be the electronic ignition coil (stick coil for the 990?) to the spark plug? Maybe the connector to the stick coil has come loose? You have no spark to either cylinder? I suppose there would be 2 of them since it's a twin. Odds that both went bad at the same time would be very remote I'd think.
I have no idea how you'd check them to see if they were damaged on the reversed battery polarity
I'm not sure about the 990, but when I was pulling the spark plug on my 690 I removed the 2 wires from the ignition coil to make it easier to get the plug wire off. I could see how those wires/spade connectors could work their way loose. I know if my 690 won't start some day that would be one of the first things I'd check.
Hope you get it sorted.