Picture of my last two cars
Left side: my current 2000 Honda Insight. Right side: not mine, but looks like my previous car - a 1988 CRX HF
The Insight was an extreme makeover of the CRX HF that replaced aluminum with steel, and 1.5 ICE with a 995cc hybrid engine.
It's going to be hard to get more power AND increase the mpgs, unless you are going to add a turbo or maybe do an engine transplant and replace the OEM with a TDI.