Small Engines (Lawn Mowers, etc.): Electric blade engage, bridge rectifier, electric clutch

Question
Hello, I asked this question of Eric Jones but the site would not let me ask a follow-up as it says he is maxed out.  A friend of mine had an 18.5 horse B&S engine seize on him.  I replaced his engine with an older 12 horse B&S.  Everything matched with the exception that his motor was equipped with an electric shut-off on the bowl of the carb.  The older engine starts and and will drive the mower but the electric clutch for the blades does not engage.  Mr. Jones stated that maybe the older engine is not putting out the right amount of power and suggested that I send the numbers of both engines for comparison.  The 18.5 horse engine has a Model # 31P777, Type 0348E11, the 12 horse has Model #284707, Type 1026-E1.  His 18.5 horse motor has a rectangular box on the side of block with two wires attached.  My old motor does not have that box.  The main wiring harnesses matched with the exception of the two wires going to the fuel cut-off in which I simply capped off the hot wire and attached the ground wire to the motor.  I know this is a long question, but since I could not reply to Mr. Jones, I had to put more into this so you would know what is what.  Just trying to help a friend who can't afford buying a new mower.  Thank you for any advice you can give.

Answer
The square box is a bridge rectifier.  It rectified the voltage coming out of the stator.  All stators produce AC voltage only.  Your battery and all the electrical accessories run on DC voltage. Hence the rectifier is 4 diodes aligned to make the DV voltage go to different parts of the electrical system.

But to answer your question about the electric clutch.  The PTO switch is a double pole double throw switch.  It has two purpose.  One to operate the clutch and one to work with the safety system.  Both have to work. If there is a diode in the wire coming from the engine you could use it to run the clutch.  If there isn't you cannot.  

What you may have to do is take the stator ring off the old engine and mount it on the new engine.  Hook up the box and you should have the same as the other engine.  The stator in under the flywheel.  Most all if the bolt patterns are the same. Look at the flywheel magnets under the flywheel are they the same?  If not see if the flywheel will mount on the other engine.  Again the hole should be the same.  Let me know if this will work for you