Clips won't play in FCPX or QT, says missing plug-in, which plug-in or workaround?

Whoa... I think Apple discontinued support for Motion JPEG quite awhile ago.

I don't know if this feature is still available, but if you right click on the video files in the Finder and select Services > Encode Selected Video Files you'll get a popup to Encode Media. This will allow you to convert your original to a format newer OS's will support.

I would recommend selecting 1080p or Apple ProRes. I don't think the aspect ratio/original size of the video will be changed even if you select the 1080p setting - you'll get an H.264/AAC file. ProRes can be a little "expensive" in terms of used disk space... but it's up to you and you don't really get a *better* version of the original... it will look pretty much exactly the same with either option.

If you do not have an option to Encode Media in Services, you'll need to resort to a third party software like FFMpeg, VLC or Handbrake.

I think Apple completely removed the Motion JPEG codec after Catalina. They removed mjpeg sometime during Mojave (which I am still using) but provided the method I described as a way to update older video codecs to something Quicktime compatible for a period of time after they pulled support.

You're probably going to need Handbrake (probably the easiest to use from the other options.)

https://handbrake.fr/downloads.php