Check out www.openppg.com. It's an open source electric paramotor. Not sure how much power they are putting out, but they claim up to 1 hour flight time.
Lithium ion batteries have high specific energy and lipo have high c rating (high power). Is it possible to put lipo and lithium-ion batteries in parallel if they are at the same voltage and protected by charge control or BMS circuitry? This would give you a battery that has high energy density...