Minnkota sells a bunch of electric trolling motors that would work, but
you have to deal with a battery which may weigh 50# or so, much heavier
than the 4 hp Johnson, (I have one, a 1987 model that weighs 33#. I love
it!) although not quite as awkward to handle. I once read something that
said 15# of thrust is approximately equal to 1 hp, but then a whole
bunch of posters chimed in with the information that thrust and hp were
as different as apples and chipmunks, so.....

I had a 7' Livingston that I am happy to be rid of, as it was too heavy
to handle and tended to like to take on water over the bow when power
was cut. I replaced it with an inflatable, which runs a little bit
wetter in big waves, but I will take it out in stuff I wouldn't even
consider doing in the Livingston.