First, there is a difference between 24fps and 24p (which is actually 23.976). I’m assuming you mean 24p for your new footage.
How were your old newsreel footage clips digitized? If they were transferred “properly” It may be possible to remove the pulldown to restore the clips to progressive 24p. Happy to describe the details of exactly what’s going on here, but probably not necessary. I abandoned apple editing software many years ago when Apple abandoned fcp7, but this used to be possible in Cinematools which was part of FCP Studio. It may still be possible in Compressor, but you’ll need to do some research on that. It is a feature in AfterEffects if you have access to it.
Removing Pulldown will give you better quality than just deinterlacing and retiming. Also, scaling the SD material in compressor with the best quality options enabled will probably do a better job of scaling than FCPX albeit with a much greater rendering time.