Hey don_patrick,
When my parents got me "involved" with installing their bathroom fan in their house a while back, I got the jist of the in's and out's of bath fans. I did some research at my store before putting it in, and here are some options that may work for you.
Option 1: Saving the fan w/ replacing just the motor
Hopefully you will have access to the attic so you can see the release tab as it maybe better accessed from that angle, depending on your model. This option of coming in that way makes it easier for you to see any additional screws or tabs. Plus, you can clean out the area of dust that has collected over time while you are up there. If you cannot get into the attic, you can attempt to check the manufacturer's model # and I can get you an online link to the manual. When you do get it out, I recommend getting an upgrade kit that we sell in stock that works for most models:
With option 1, its going to be a bit more time consuming project than a money one, since finding a replacement motor may not fit into the existing box. Be sure to check with a licensed electrician or HVAC repairman for any questions we didn't address here or for specific reconfiguring.
Option 2: Replace the entire bathroom fan
Sometimes its just easier and cheaper to replace it outright if option 1 doesn't work out. Plus, you can save time knowing you haven't altered the housing by trying to take the motor itself out with possible damage. Usually this is the way most of my customers I have spoken to over the years do, as sometimes if you can find a replacement part for your fan, it may not be the exact fit. Come down to your local store and our associates in the Kitchens/Bath department can get you the right replacement fan and/or parts. Also, check out the link I provided that has a lot of online choices as well here.
The only drawback to this is if you have bought a smaller or larger unit, youd need to reconfigure the duct and drywall in the ceiling so that the unit can fit snug and safely. However, the end result means you can have a perfectly working bath fan for years to come.
Hope this helps you out and any further assistance let us know
Regards,
aboveaveragejoe
For anyone still needing help with this......
Had the same issue - my motor housing did not have a screw or any clips I could see.
Used this video as a guide.
After I REALLY dusted out the motor housing I was finally able to see the clips - 1 large one in the center of one side and 2 smaller ones on the opposite side.
New motor unit cost $15 and took about 10 mins to install (after finding the clips, of course 
).
