The Broan motor part numbers are exactly the same (99080166) as are the impeller wheels (99110446). I don't know the difference between BP28 and BP51 kits but it could simply be included accessories to make it fit Model 694 and 695-A.
I have two 671-A units in small bathrooms (40 & 45sqft) that use BP28 kits. I wish Broan made different speed motors for the same application. I'd like to use the recommended "99110446" impeller wheel at a lower sone without using a fan speed controller.