Thanks for reaching out. As you’re using AMP in reader mode you could add your own search box to your legacy reader mode AMP theme. An example basic search could be adding using the below:

<form role="search" method="get" id="searchform" class="searchform" action="https://www.cacanhmini.com/">
<div>
<label class="screen-reader-text" for="s">Search for:</label>
<input type="text" value="" name="s" id="s" />
<input type="submit" id="searchsubmit" value="Search" />
</div>
</form>

To customize the legacy reader theme templates you simply copy any of the AMP template files into a newly created amp folder inside your active themes parent directly, and customize any of those PHP files within the amp folder. These will overwrite the plugins legacy reader mode templates.

As an alternative have you tried using installing and selecting any of the AMP compatible themes and using them in reader mode as an alternative to the legacy reader theme?

No problem at all, be sure to reach out if you have any further questions. After testing out the AMP plugin and comparing performance between your AMP and non AMP URLS be sure to share your thoughts on the plugin so far.