I hate to be the one to open the floodgates, but this is just patently wrong. Incredibly difficult to port, sure; incredibly unlikely to be ported, sure. But engine recreations exist for a number of games whose source has never been released. NXEngine for Cave Story, OpenJazz for Jazz Jackrabbit, OpenTTD for Transport Tycoon Deluxe, a whole host of games included in ScummVM are not working from source; hell, even Sonic the Hedgehog is well enough understood that Stealth ported it to Game Boy Advance.
I'm loathe to post this because I don't want people to get the impression that this is easy, these engine recreations have taken years and years of work, none of them has happened overnight. But with extreme knowledge and dedication, often by multiple people, all offering their time out of passion and the goodness of their hearts, it is absolutely possible to clone an engine and port it to other platforms without any source release for the original game. It's fair to say that if an engine recreation doesn't already exist for a game, it's vanishingly unlikely to appear on the (New) 3DS, because the system will be out of date by the time such a project got off the ground, but to say these things are impossible is to ignore the incredible efforts of hundreds of talented developers.