That's easy … shop SBO for halyards. Personally, I would look at the high-tech double braids. I have Warp Speed halyards but I think the Validator or MLX are also good choices. For strength, 5/16" (8 mm) will work fine. 1/4" is too small to handle nicely, I think. If you look at XLS or Sta Set, then figure on 3/8".
Read a little bit on-line about those products and you will get an idea about what you prefer. These are most-often personal choices. If you think your halyard is sketchy, by all means, change it out. You will be glad you did. Do yourself a favor and have a shackle spliced to one end and have the other end whipped by the store. They do a great job.
Dave is showing you the standard product specification for your boat - that's the easy way to go about it. You can have the halyard made to your own specification and choose your own line diameter if you wish. I notice that the line diameters aren't shown on the product selections. I would ask about that so you know what you are buying. Or order the halyard with the product, length and diameter that you choose. Either way … it's your choice.