The magnifier icon in the Viewer Display is an SVG with its path defined in the exported HTML file. Here's information on editing an SVG path:
Alternatively, depending on the content to magnify, you could group it and put it into an MSO with the original scale in the first object state and a larger version of the content in a second object state. Then instead of using the Viewer Display zoom, you could create your own button and add a Go To Next State action in the Buttons and Forms panel (Window > Interactive > Buttons and Forms) to toggle the magnification. Here's more information:
