Well, 20Ah is a battery term, not a controller one, so that alone doesn't make any sense.
As for what's compatible, we don't know anything about your bike or your usage, so we can't tell you what might work.
What controller, display (if any), throttle, motor, battery, etc does it have now? Complete details on each one are needed to be sure we recommend something compatible. If it's an unmodified prebuilt bike, then a link to the page it was bought from might have some useful info.
What is the specific reason the controller needs to be replaced? Knowing that may also help us help you ensure all the necessary parts get replaced along with the controller. (you'll probably have to replace the controller and display as a set, if it has one, and any other controller-dependent hardware. Anything that depends on the original controller to do what it does won't work anymore).
What is your usage and riding conditions? (to be sure you get a controller powerful enough to do what you want).