I used mag wheel cleaner on a carb and an aluminum transmission case already. The kind you spray on and later rinse with water. It has some type of acid in it.
For parts like alternator cases though, a bead blast (or walnut shells) will make it look like new! The downside is you would have to take it all apart to do that.
Option #3 would be to hit it with a wire wheel on a drill, then spray it with aluminum paint such as Eastwoods Alumablast.