Loose bolts will do it lol.I forgot that you did completely dissemble and rebuild your engines.Briggs is pretty vague on torque specs for the Flat head engines.
1/2" bolts with lock washers go to the same torque specs as the Oil sump bolts 140 in. lbs. or 11ft lbs 12" bolts with integral washers like your starter bolts go 200 in. lbs or 16 ft lbs.
I would change out those screws on your stator for the bolts to keep them from backing out again.Never liked the screws.You can't feel how tight your getting them when your squeezing the driver.If you could calibrate a Drill driver and figure out how tight they tighten screws then you might be able to get them all evenly tightened.I've had a few of those screws back out completely and get into the magnets and make a mess of them.
Find a couple of these puppies that fit and tighten them up.I don't use loctite on any bolt that goes into aluminum.Had a couple bad experiences where the loctite gauled up and pulled the threads.even the blue light duty loctite does this in soft aluminum.Even had connecting rod bolts gaul up with small dot of loctite applied.I read later that the loctite heats up very hot as it cures and it etches into the metal.Not something ya want on aluminum lol.
Was saying about the newer vanguard twin stator because there 10 13 or 16 amp depending on the size of your magnets.Less stress on the system makes them last alot longer.