Mailmunch plugin is now compatible with AMPforWP. It requires an Email Opt-in Forms for AMP  addon to enable it.

What features are compatible at the moment?

1. Automatically Show optin Box in content
2. Above or Below or Between the 50% Content
3. Post Types Support
4. POP-UP FORM
5. MODAL FORM
6. Exclude Optin For Specific Posts/Pages
7. Shortcode Support

If there is any feature from the Mailmunch that is not available, then you can request it from this support page.