Low Resolution Fullscreen

Trying to run project 64 2.3 (also tried with 1.6) on a fanless mini PC using an Intel Atom x5-z8350. It runs just fine at 1280x720. At 1920x1080 however, I'm getting slowdowns.

I'd like to run it in fullscreen at 1280x720. Unfortunately this only renders it at 1280x720 centered with black borders, as opposed to 1280x720 stretched to the borders of the monitor.

Is there any way I can achieve the desired result?