See How to stop the Opus image viewer from being used by default in the Viewer and Viewer Plugins section of the FAQs.
You can do it via AutoHotkey, if that helps.
Other than that workaround, the options are:
[ul][li]Page Up and Page Down[/li]
[li]Space and Backspace[/li]
[li]Mouse-wheel up / down[/li]
[li]Left-click[/li][/ul]