Small Engines (Lawn Mowers, etc.): Electrical blade engage, briggs and stratton, pulley system

Question
Friend of mine had his 18.5 horse engine seize.  I replaced the engine with an older model 12 horse Briggs and Stratton.  I used his pulley system on the older engine as the shaft coming out of the engines were the same.  His mower has an electrical blade engagement system.  The mower will start and run and drive but the blades will not engage.  The motor does not quit when engaging the blades.  All wiring to the electric clutch is plugged together tight. Is there something about how the pulley system goes together that I may be missing?

Answer
I'm wondering if the 18.5 horse engine had a larger alternator?  I'm guessing it did.  Do you know if the old engine had a regulator?

Did you check the charging system on the 12 horse to see how much voltage it is producing?  It may not be producing enough power to engage the clutch.

Send me the model nubmers of the old and new engines and I will try to see which alternators they have.

Eric