Yes, you can use the frame as a coomon ground. If you're using a battery simply run a wire from the battey's negative post to a bolt or screw that is threaded into or against the frame and each light/device the same way. The positive side of the battery will then go through a switch/button, whatever, then to the device.
Each place you ground must be a metal to metal attachment which will give you a good electrical connection. Paint will need to be removed but just enough to allow attachment of your wire/s. You don't need to remove a lot of paint.
As for the engine, you didn't say which engine you have, but in the case of a Chinese 2 stroke the engine case is a ground unto itself and needs no electrical connection to the frame. The magneto is directly grounded to the engine case where the mag coil is attached. If you're using a kill switch it should be wired to the blue and black leads from the magneto and needs no connection to the frame. Don't use the white wire in your kill switch circuit.
Tom