story identification - Tracking down an old sci-fi book

I'm trying to track down an old sci-fi book I read when I was a teenager.

The book was at the Cottage Grove, OR Public Library around 1982-1985.

The plot followed a brother and sister who were scientists or spies, or both.

The brother once used a series of pills to change his weight, height and appearance.

Towards the end of the book, the brother takes a space ship that his sister invented, removes a governor so that he can travel faster than the speed of light and goes back in time several hours.

This is really vague, but thanks for any leads you can share.

Answer

The Space Eagle: Operation Doomsday, (1967), by Jack Pearl.

Julie and Paul are twins, and even though she's a brilliant scientist, Paul is constantly questioning her value as a person. And he wins every time, and makes her feel like an idiot. Julie just happens to be working on a new super-fast spacecraft – the SWIFT – a vehicle designed to travel at the speed of light.

...

Paul and Julie spend 96 of their 100 hours getting the SWIFT ready. The last thing Julie needs to do is install a governor on the engine, so that Paul doesn't go too fast.

...

Paul adds in an "instant plastic surgery" kit developed by the Girard Foundation. It's a pill and some ointment, and after you take it, your "muscles, flesh, and bones [...] can be molded and reshaped much as a sculptor shapes a clay figure." There's also pills to change your skin color, and some sort of compound which causes "the human body to burn off calories at an enormous rate in a short time" -- so you can drop 100 pounds at once.

...

Paul seems to discover that time reverses itself when you exceed the speed of light.