Yeah, take the PAL DVD9 720x576 video track, change the framerate from 25fps to 23.976fps, upscale that, then pair it with the NTSC DVD9 Dolby 5.1 audio track. Upscaling based on the downscaled NTSC release just seems like a bad place to start.
There’s already an MKV floating around made from the PAL DVD9 that basically does everything I mentioned minus the upscaling part.