kawasaki 650 backfire

I'm at a big loss here my uncle was plowing the driveway with our 2006 kawasaki 650 and the low oil light started flashing and the quad shut off he is not mechanically inclined or know much about engines and he went ahead to add oil on and incline and over filled it and locked it up/hydro locked...

I just changed the drive belt on my 650 Prairie and the first time I tried to ride it I got a serious power loss and backfiring at about 1/4 throttle and beyond. I had someone tell me a new rectifier was needed but, am not convinced that is the problem. Any ideas? Horseman 84