When you shut off the engine, the piston stops opposite the compression stroke, and it's difficult for the starter to move it past top dead center. A weak battery, like the 4AH that comes with a 50cc, can't push the cold piston over top dead center. It just clicks and does nothing when too cold, like in winter. If you nudge the piston past TDC, it will start fine. Or switch to a 7AH battery (which might be bigger than your battery box). I think you'll find if you nudge the kick starter just past the compression stroke, it'll start every time with the electric starter, even in winter.