As Jeff said it is possible that the engines are different enough by having different length crankshafts or even different mounting hole position to make up the difference. What you need to check is the orginal position of hte carrier before the change and after then you would know if it is the mounting that made the difference.
I recently had to replace an old Briggs L head horizontal with a new Briggs OHV horizontal on a log splitter. Due the positioning of the mounting circle (actually 4 bolt square mount) for the pump I ended up having the shorten the crankshaft by 5/8" so the LoveJoy coupler would fit correctly. Another option was a custom spacer but cutting the crankshaft was the better less expensive option.
