You can use any Wacom compatible stylus to replace the existing stylus and the third party stylus will probably perform better than the original based on other owners' reports.
Check these comprehensive three parts in-depth review video of the Onyx Boox Note Air by a reputable eBook reader reviewer [1].
My main complain is that it has a rather limited built-in web based app to transfer from your laptop/PC/etc to the device directly via WiFi but not the other around. If you want to transfer wirelessly from the device to your laptop/PC/etc you need to register with Onyx's portal and sync from there, thus it's a more cumbersome to use. Normally I just email the document to myself because unlike Remarkable's devices it has Android OS and normal browser being installed.