The thermostat in this engine only fits in one way, it's impossible to insert it any other way:
The port was fairly clean, but there is a little ball or something up in there that was "stuck" in place. Spraying with some CLR and prodding with some wire got it freed up and was passing air freely. I'm not entirely certain how that part is supposed to work, there doesn't seem to be any info on that fitting or it's function anywhere I've searched.