I had a 1961 900 sportster with right side shift back in high school, fun little bike. First off you can technically go to 70 over on those thick wall cylinders but I wouldn't. You'll spend the money for new pistons and the machine work and have a good chance of distorting the cylinder wall with use. If you are rebuilding the motor I recommend getting new jugs. I belive that you can still get oversized aftermarket jugs for those old sportys but it is getting harder and harder. Impossible from the dealer. As far as your displacement goes I would have to measure it to tell you where you are at.
If you are looking for more power there are a few things you can do with out boring or replacing cylinders. Either have your heads flow ported by a shop with a flow bench or do it yourself with a die grinder. I am in the process of building a web site that will have project details on this and other things for the do it yourselfer ( www.bikergarage.com ).
Another thing you can do is get rid of that crappy bendix/zenith carb. Now I know that some of the old timers love these old carbs but they just don't perform well, they have a terrible accelerator pump design.
Another thing you can do is change your sprockets this will give you more bottom end which I am sure that is what you are looking to do since these bikes don't do very well over 70 mph. You can only change the front drive sproket because the rear is incorporated in to the brake drum.
Hope this helps if you have questions or need some hard to find parts send me an email