There's a fatal flaw in your calculus: only 20-30% of the energy in fuel turns into shaft horsepowers, whereas electric motors have 90% efficiency.
But there is no argument that fossil fuel contains more W/kg than batteries at present. That's for certain. However on the other side of the spectrum, there is also no argument that electric motors can produce more W/kg than any other prime mover. They can't be beat, not even by turbines.
So by getting rid of all the dead weight you're lugging around in the form of your combustion engine, taking into the account that it's also just 30% efficient, the gap is closer. A 200lbs Lycoming could be replaced by a 20lbs brushless with the same output. That's 180lbs you can now use for something else, all things being equal. Like a battery or a hybrid APU.
It will take time before energy storage can match that of fossil fuels, but when you take into the considerations all the stuff you don't need anymore, then the gap you need to bridge is less.