It is inevitable that when you scale an image, quality will go down. You are losing resolution, using less of the pixels from original clip to fill the same amount of screen.
What are the specs of the source clip? For instance 1080p, or what? There is a trick you might benefit from - edit the clip in a sequence with lower resolution, for instance edit 1080p clip in a 720p sequence. The clip will be larger than the "frame" you are putting it in. Now you have some extra pixels to play with. Use Adobe Motion to adjust scale. You will be able to zoom in some without losing any quality since you have more pixels than needed to fill the 720p frame. Once you zoom to a certain point though, you do then start blowing up the image and again losing quality. But this will afford you some leeway. Then export as 720p.
Putting 1080p clip in 720p sequence, setting Scale to 67% you will see entire original clip, then you can zoom in as far as 100% with NO quality loss. Anything past 100% you are starting to blow up the image. You might find that up to 110-120% looks acceptable, depends on quality and detail of original clip.