How to open .MPG file from old iPhoto library with Mac OS 13.3.

Not all video formats we could use in iPhoto are supported any longer by macOS 13 Ventura. It will depend on your particular mpg file, if Photos will be able to open it. (About incompatible media in iMovie for macOS - Apple Support) If you still have a mac with macOS 10.14 Mojave, use it to check your videos for compatibility. when you import an unsupported video to Photos, you will get a warning. In that case convert the video to a format, that will be compatible with Photos on Ventura. I have converted all videos to HEVC or H.264 before the upgrade to macOS 10.15 Catalina, when Apple removed the support for older legacy video formats. If you no longer have an older Mac with Mojave or earlier and cannot borrow one, you may need to use third party apps to convert older videos. I am using Handbrake for this.

yes

Import the file file from the old iPhoto Library to Photos and play it from there.