Legendre is correct, it looks like an early 70s model S 3800. If you would like to know the exact year, let me know the seven digit serial number _stamped_ into the top rear of the motor. It will probably start with a five. Don't be thrown off by the 14-3-1966 date cast into the top of the motor, it is the date of approval of that type motor. The seven digit frame serial number stamped into the right side of the frame downtube near the neck weld would be helpful, but not necessary.
To try to get your bike started, you will need at least the fuel pump, fuel pump diaphragm (sometimes incorrectly called a membrane), and the fuel pump seat. That's what is missing from the front of the motor.
Go to: http://velocruz.com/store/carburator.htm You need ref. 15, 16 and 17. A problem will be that I only see two of the four screws that hold the pump to the crankcase. These screws are M4 x .75, not easily obtained. Perhaps one of the forum readers can help you out with two screws. They are available from France. You might be able to retap the holes M4 x .70 and use the screws from Velocruz, but I have never done that.
You are also missing the flywheel hood, and motor hood which houses the headlight. The flywheel hood is available from Velocruz, no problem. The motor hood is also available there. The headlight that they sell will fit, but you will need an adapter to install a smaller bulb, because your electrics will probably not power that bulb the way they are.
The taillight is not original and looks like an Aprilia unit. These units were added by some dealers or distributors usually to make the bikes a little more legal. Yours may have a crude brake light system added to it, also, that only works when the rear brake is applied; can't quite tell from the photo.
There were probably Solex stickers on the sides of the frame downtube originally.
Attached is a photo of basically the same bike.
I'd be happy to try to help you any way I can.