If you have Photoshop 2020 you can try this trick:
- open your movie and convert it into a smart object
- apply a field blur of 1 px (= no blur) from the blur gallery
- then you can add a very realistic simulated grain and change a lot of parameters (amount, size, roughness, color, highlights)
By comparing with a real 35mm scan (which I’ll provide to you), you will be able to change the parameters and get a very natural grain, quite similar to the original one.