Is it possible to build an ultralight helicopter with electric motors as its main engines?
From a quick Google search, you can find that Sikorsky Firefly and Tier One R44 are two examples of eHelos, conventional helicopters converted to electric propulsion. They both fly around 20-30 minutes (though the Firefly never flew).
As for building an electric helicopter, I believe off-the-shelf components (such as Tesla motors and batteries, available from parts breakers) have enough performance for an eHelo. The actual configuration of the aircraft (single main, coaxial, multicopter etc.) depends on the manufacturer's expertise on rotor blade hubs and/or distributed electric propulsion. So unless you work for Kamov, a coaxial rotor hub will be prohibitively complex (thus expensive) for a prototype ultralight helicopter.