Sorry for late response.

This code will remove the email content limit.

add_filter( 'wpforo_email_notification_post_body_length', function(){ return 0; } );

You can insert it in your active themes functions.php file or use snippets plugin.

Solution by inserting the code in Themes functions.php file

Add the below code at the end(bottom) of the current active theme functions.php file

Option one: open functions.php file from wp-content > themes > current active theme.

Option Two: Go to Dashboard > Appearance Theme File Editor, select Theme Functions(functions.php) file from Theme Files Bar.

Please mention that the code can be lost after updating or changing the theme.


Solution by using Code Snippets Plugin

Install and Activate Code Snippets plugin(https://wordpress.org/plugins/code-snippets/).
Go to Dashboard > Snippets > Add New.
Insert the code in code field:
Select Run snippet everywhere Radio and save the snippet.
Check that the new created snippet is active from Dashboard > Snippets > All Snippets.

Is there a way to make to show the full post of topic and Reply rather than an excerpt only in the email notification?

This will help users know whether they want to reply or not on the topic or reply before going to the forum. 

Thank you always for a great support.

Mike