If you have a 3d printer then it's possible to make a movement holder to position the movement correctly inside the case. I've done this for a custom watch I made. I also printed a dial is it was a skeletonised movement. It takes some time and effort to get right, here's my prototypes...
Thank you all very much for your helpful replies.
I feel silly for not checking ranfft first, not the first time that website has gotten me out of trouble.
do you believe the inner hook would be on a tighter coil?
I might just try anyway. I'll certainly try and check and compare the dimensions of the original.
I'd have to check, but I think it's a video that I've seen. There are a few of these handy little watches out there. Maybe I could post my own video here. I'll have to film it in action! That'd be a cool thing.