The code in this repository is under the Belay Source-Available License 1.0. LICENSE is the whole of what is
granted, and it grants a great deal: copy it, change it, ship it, give it away.
The one thing it does not grant is selling it – see the "NO SELLING" clause in
LICENSE. Belay is free, and staying free is deliberate.
What that licence does not grant, because a copyright licence never does, is the right to use somebody's name for your product. So, plainly:
"Belay", the Belay wordmark and the Belay app icon are not covered by the licence. They belong to PerfectoWeb.
Fork it, rename it, give it away. Genuinely. Take the code, call it something else, put your own mark on it, ship it to whoever you like. You need nothing from us beyond keeping the copyright notice, the credit the licence asks for, and the one rule that travels with the code: you may not sell it, and neither may anyone downstream, without written permission from PerfectoWeb.
Do not ship it as "Belay". A build that is not ours should not arrive on somebody's Mac wearing our name, because the person installing it cannot tell the difference and will come to us when it misbehaves. That is the whole of the objection, and it is about them rather than about us.
Say where it came from. Not a legal requirement, just the ordinary courtesy: "based on Belay" with a link costs you a line and tells your users something true.
Talking about Belay is always fine. Reviews, comparisons, tutorials, "works with Belay", a screenshot in a blog post. Nominative use needs no permission and we are not going to pretend otherwise.
Ask. https://github.com/PerfectoWeb/Belay/issues or the address on https://perfecto-web.com. For anything that helps people find the real thing, the answer is likely yes.