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.
My suggestion would be to use Topaz Video Enhance AI to up-convert and deinterlace the clips. Take the 59.94p result into FCP and drop them into your 23.976 timeline. It will drop frames to stay in sync, but the temporal cadence should be acceptable given that you had 60 frames to choose from (24 from 60). If the speed of the clip doesn’t need to match (no sync dialog, for example), interpolate the source as 50p and FCP will skip every other frame giving you perfect cadence.
Hi Michael, thank you very much for your help as well. I purchased my WW2 clips from Buyout Footage on Mini-DV tapes (480i29.97). They have a number of titles in HD 1080p24 but it was expensive enough to acquire them in standard video. So as I can’t afford to buy the HD version, I initially upconverted them to 720p59.94, but I decided instead on 1080i29.97. I’m using a Kona LHi card as my hardware for upconverting. The only choices it gives me for that are 720p59.94 or 1080i29.97. I don’t have Compressor yet, but I’m an Adobe CC subscriber and I use AE regularly. I’d love to be able to turn those WW2 newsreels into 1080p24 format. Would Compressor or AE be better at doing that?