[imFiles]

This one works for bbpress: https://wordpress.org/plugins/contextual-related-posts/ if you want only related topics just go into Contextual Related Posts Settings and on the List tuning tab and on Post types to include select topic only. One thing it doesn’t have an automatic option to add the related items, so you need to added them either manually or the best way is with a fucntion.
I use this one to add it after each topic:

//add adsense after topic
add_action( 'bbp_template_after_replies_loop', 'rew_other_topics' );
function rew_other_topics () {
	 $topic_id = bbp_get_topic_id() ;
	 $forum_id = bbp_get_topic_forum_id($topic_id) ;
	if ( function_exists( 'echo_crp' ) ) { echo_crp(); }
}