PD is already upscaling the zoomed in area to fill the 1920x1080 screen. Higher quality source clips (those with video bitrates of 50+ megabits/second) will lead to higher quality upscaling, but in this situation the choices are either to not zoom in so much or use a paid third party upscaling app like Topaz Video Enhance AI.
In the latter, you can upscale your source clip to 4K or 8K which would give you plenty of detail when zoomed in at 1080p, but you'll need a powerful GPU and lots of time to process the clip.