750 Atlas Re-build

Do you have any reason to suspect the engine needs to be stripped down?  A quick way to end up with a permanent box of bits is to dive in and take things apart for no good reason.  I would drain the sump (it will probably be full of oil from the tank), replace the filter (if it has one), check tappet clearances, fill the oil tank, squirt some oil down the pushrod tubes, and get it going.  After it settles down you will get an idea of the state inside.  Don't cure a smoking engine unless the engine smokes.  Nor replace bearings unless they need replacing.

What you do with cosmetics  including oil leaks, will be up to you.

Other opinions will no doubt appear.  But a running bike is better than a basket case.  And if it presents problems, they'll appear when it starts being used.  Then you'll put your effort into something worthwhile.

Good luck.  And go along to your local Branch.

Agree with David. Replacement parts are mostly aftermarket and sometimes  very inferior to orriginal. fitment. And not unknown to cause expensive damage. Throw nothing away. You may need to re-fit it or use as pattern. Unless you have experience with these motors you are likely to have a steep and expensive learning curve with no bike to ride for months. 

My advice is if it aint broke don't fix it. These old engines are tough as nails. Get the old oil out, clean the carbs and the inside of the oil tank, check the tappet clearances, fit some new plugs and make sure the ignition timing is correct and fire her up! If the oil is returning to the tank and it doesn't sound like a bag of nails it's a good un.

I did just that with my Atlas which had sat untouched and unloved for 50 years! It fired up on the second kick and it's been going strong ever since.