Since you've decided not to use the larger engine, this information is kinda moot, but nevertheless here it is. The Brison 3.2 is 5.359 inches from the rear mount face to the prop hub face. Height is 5.5 inches above the crank center to the top of the spark plug and width across the cylinder fins is 3.6 inches. The carb sticks out from the centerline 3.5 inches. The mount is a rectangular plate 2.5 inches high by 3.5 inches wide, on the centerline of the crank.

If you go to http://www.brisonaircraft.com you can view and download front and side views of their engines. You can print these and then take them to a print place and Xerox them to 100% and use them when laying out the engine on the plans.

If I were Balsa USA I would not recommend this size engine on the plane, because I would not want someone to get pissed at me if it had a structural problem, or even worse, sueing me. Better safe than sorry. But I am certain that I could reinforce the airframe to make it acceptable at a low cost and little added weight.