My guess is that the broken cap screw is an M8, but it might be an M10. Not sure what the length would be, but a guess is that it will be around 30-40 mm.
Any good hardware store or auto parts store should normally stock them. The thing to be careful about is that you get one with the appropriate strength rating.
If you have the broken screw cap end it may be marked. If not, you might try to locate a grade 12.9 screw. That may have compatibility issues with the alloy post.
Hopefully the manufacturer will tell you the size and grade of screw to use. If they don't, use a 12.9 or 10.9 screw and some anti seize on the screw. Oh, also maybe a torque wrench🤕