Is CiviEngage available with Backdrop CMS?
It seems that the long-term plan is to turn CiviEngage into an extension for CiviCRM (see here and here) and deprecate the module. I don't see much recent action on that front, so you might be right that a simple port to BackdropCMS from the Drupal7 module would be the quickest way to start using it in the meantime. In short:


If you are able to help convert to a native CiviCRM extension (by coding or sponsoring), that
would be a great place to start.
If you can't do that but can do the module port to Backdrop CMS, that should be quite painless (he says without having looked at the code directly).
Beyond those, you could also submit a module port request for Backdrop CMS here. (I'm guessing it won't get into CiviCRM's core Backdrop implementation since the plan is to phase that into an extension, but it could be ported and hosted on BackdropCMS's contrib section in the meantime until such an extension exists).


UPDATE: It seems that CiviEngage may be ported to Backdrop and included in the Backdrop version of CiviCRM, unbeknownst to me. I have not tested it but give it a shot. 
https://github.com/civicrm/civicrm-backdrop/tree/1.x-master/modules/civicrm_engage