Don't use oven cleaner it has lie, will eat off paint in the over spray.
Did the mag wheel cleaner have phosphoric acid.
Your aluminum is oxidizing the acid gets rid of that and makes a great paint prep.

 

I use bleach white on a rag then wipe it clean with detailer to remove the bleach white.
Really bad stains is usually in the paint, chemical reaction. These are not really stains it is more of a paint change due to a reaction from something. The silver engines are the hardest to keep looking like new. Back is easy, pig spit.

 

Ok I am at a computer have a little more time.
Different years and different parts of the motor have different treatment on the aluminum.
Some parts like the fins are bare aluminum.
Other parts have a clear coat, and some are painted silver paint.
You can tell by taking a wax on a clean cloth and seeing if the wax/rag turns black, then it is not clear coated.
If it does not then it is cleared or painted silver.
Most of the engine is cleared on most HD's as well as the front forks.
YOU do not want to try this if you are going to paint the motor since you do not want wax before you paint.
If you are going to paint and you have used oven cleaner you should have it clean enough for the paint to stick even if it is clear coated or painted.
If the white stuff is under the clear then the only way to get rid of it is to strip the clear and why worry since you are going to paint anyway, the paint should stick due to the caustic nature of the lie, and how the clear will be etched.

 

Sounds like you "burnt" the finish....