You could use a product that SFMC used to sell actively but is shy of these days called "Live Images". It has a heap more functionality to it and wasn't particularly cheap, but it can swap out images at open time based on a bunch of criteria including a specific date and time.
Other than that, if MotioMail can't do what you're looking for within the image, you can role your own version of Live Images using Landing Pages and embed the URL of the landing page as the image. The landing page has a simple switch statement to test the current date and time. If the time you specified has not passed, you redirect to the MotionMail image. If it's passed, you respond with an empty response using Server Side JavaScript. The issue with this is you use a Super Message each time the landing page is rendered - each time the email is opened and images downloaded.
You can't use AMPScript/SSJS to alter the HTML in the subscriber's inbox at open time, so you'd need something like MotioMail to place some other content around the timer in the image itself to explain what the timer's for.