Repairs 95 dixon ZTR 4421 Battery not charging

Hello!! I had swapped the original engine Briggs and Stratton 13HP to a 12HP B&S Power Built and now it doesn't charge the battery. I noticed on my old engine there is a small square solid state device attached to the flywheel cover which isn't there on the 12HP engine cover. Is that how it charges the battery? I looked under the flywheel cover and all I see is the magneto and nothing else. So the wire that connected to that device isn't hooked up, It has a red and a yellow wire that comes out of the solid state device. I would guess that one comes from the key switch and the other goes to ground?

Thanks KEITH

Repairs 95 dixon ZTR 4421 Battery not charging