Thanks Weezer, and thanks again everybody!!
The part came in today, and since there's no time like the present, I hauled a bucket of tools up on the roof and got the job done.
For the "next guy" who has the same problem, let me answer my own original questions
NO, there is not simply a snap on plastic cover to replace the one that fell apart
Since its riveted to the metal frame that cranks up and down, the entire unit needs to be replaced. Nothing is simple these days....... but this job wasn't too bad. Of course I am a DIY kinda guy, and save a penny whenever possible, so I ordered the vent only, without the fan. The fan would have cost me an extra $50. (crazy right?) So with a little ingenuity, and some muscle, I was able to crack the spot welds on the old setup, and move the fan over to the new gig. Only had to drill a couple holes and do a little re-wiring. Definitely worth the extra 30 minutes of labor to save me $50 on the parts bill.
As far as the Maxx air .... dang those things look awesome
BUT, unfortunately I cannot have one. The barn where she resides when not in use only gives me a couple spare inches of clearance as it is. (That's with everything closed up and cranked down).... so I had to skip it. Good thing I checked clearance before ordering and installing, or this post might not have such a positive attitude
Thanks again for all the help. If anyone has any questions now, or down the road, I would be more than happy to help out!!
The first picture here is the old vent on the left, with the missing plastic cover, after I stripped the fan off of it. And the new one is on the right, half way through fan installation.