Hello Estella,
welcome to the forum, hopefully the guys on here can help you with any questions you may have.
The engine and frame numbers you have quoted are for a 1952 "Star Twin" A7, thats what the S in the engine number means. A very nice model to own.
The tank in the pictures is the correct one for a plunger frame bike, the ones you have seen with the hole in the middle are for the later swingarm frame models. I tried to find a decent picture of a 52' model on the net but could only come up with this one. The bike in this pic looks ok but does have the wrong kneepads.
I'm in Australia so not sure where you could access parts over your way, most parts are available in the UK but maybe Domiracer in the states would be a possibility for you. Getting a single seat will be easy, they are readily available at most shops that deal in old bike parts.
Good luck with it.