I have done several tanks with Rust911, and it works very nicely. I used a magnetic oil pan heater to keep the solution at about 140 deg. for at least a week with frequent agitation. The inside was spotless after being checked with ah endoscope.
I've used Rust911 for 5 yrs. with superb results. It is an excellent product.I just do not understand why people play around with acids when this superior product is available. Works really well to derust the inside of the gas tank. It comes out spotless.
I have the elec. wiper motor you want and its cheap, but I agree with the above sentiments. You would need to drill new holes. You are better off with an original.