What a good result!
You may recall we had a chat about this in the 200 section at the time you had convinced yourself that you would need a new motor. The consensus was that red blocks are pretty tough and it would be worthwhile just trying a new belt. I'm absolutely delighted that has worked out well.
Alan (from the 200 section).
PS. The motor sounds fine in the YouTube video. Checking the valve shims is a good idea though.