Thanks for everyone's help. We didn't think of using a longer center bolt and spacers underneath, we did go ahead and make a spacer frame out of 1 inch square tubing, to set the motor base up an inch off the deck. I welded the square tubing at the corners and drilled holes to match the bolt pattern on the motor and the deck. Everything else went right into place and the mower is now back in service with a brand new B&S engine. It started right up and needed no adjustments in order to operate perfectly. The only significant difference was in the starter solenoid. The Generac starter was equipped with a integral solenoid and with the B&S engine I had to mount a separate starter relay on the chassis. I rerouted a few wires but maintained all the safety switches. If I remember correctly the B&S engine didn't have a low oil pressure delay switch, so those wires were just not used. It took a lot of thought to be sure and get the wiring right but we did not have to buy a new harness.
So to anyone out there who is considering an engine swap: Think it through as you transfer the wiring. Common sense is your best ally.
M48TANKER