as far as mounting ideas... take a look at staton-inc.com for a rack mount.
If you want to use the existing gearbox - what is the ratio on it? 3:1 or 4:1 ?
You'll want the rear wheel to end up somewhere on the order of 250 RPM on a 26 inch wheel. 250 RPM on a 26 inch diameter wheel is apx. 20 MPH. Starting at a "cruising" RPM of 5500, you'll need a 22:1 total reduction to achieve a cruise of 20 mph. (which would let you attain a max speed of about 28.) If the gearbox is 4:1, then you will need a further reduction of 5.5:1. Since there's a 12 tooth sprocket on the gearbox, you'd need either a 66T sprocket at the rear wheel, or a jackshaft. I think staton has some right-side free-wheeling sprockets with about 20 teeth. If you went to a 40T sprocket on the jackshaft, then down to a 12T sprocket again, that would be just about right with that 20T freewheeling sprocket. Of course, a jackshaft would let you switch sides if you wanted, to get the freewheeling sprocket on the left side...
That motor/gearbox is a little wide to frame mount. It would be tough to peddle without singing your legs!