Also, it's possible to have multiple, small electric motors with no loss of efficiency. That isn't possible for a gas engine. Multiple motors could add some redundancy, and therefore reliability.
I think we're going to see a lot of interesting designs over the next few years.
That varies wildly based on what kind of engines and motors you're talking about; it's not consistent enough to be applied as a generalization, especially when it comes to aircraft (turbines are some of the most power-dense engines available). See here [1] for examples.
[1] https://en.wikipedia.org/wiki/Power-to-weight_ratio#Examples
The issue will always be that fuel is consumed and causes the aircraft to be lighter the longer you fly.
Chip Yates flew a few experimental electric airplanes a few years back. The last I heard, he had devised some sort of battery hot-swap system so he could fly from NY to England in one go. I don't think that one ever made it off the drawing board.