I have found my old gear bike out back. I wonder, could I switch the transmission for the old gear bike to the 1 speed I have?
simply put: no
not simply put: your cruiser has a narrower fork in back because it doesn't need the room for the extra sprockets, also the rear sprockets are connected to the wheel, and you'd have to replace the whole wheel (which wouldn't fit) and then you'd have no brakes
and you'd also have no way of shifting since the cruiser doesn't have mounts for the derailer, and the cables would all be the wrong lengths