Canonical makes the
.torrent file for the various ISO install images available for download. You can download the
.torrent file for an install image, as well as various other download meta-files and the actual ISO images files, on the Ubuntu Releases page organized by release.
For example, here is the link for the page of the most frequently downloaded files for Ubuntu 12.04 LTS. The first line of a "most frequently downloaded" page for a release also contains a link to the list of download mirrors and a link to a page where you can download other images, including DVDs and source CDs.
I personally have not seen a magnet link for an Ubuntu ISO image torrent on a Canonical site. I think they don't bother with them since it is debatable whether doing so would result in a meaningful (i.e. noticeable) performance improvement.
.torrent files for the Ubuntu ISOs tend to be quite small. Typical
.torrent files sizes appear to be well under 40 KB for a CD ISO and under 100 KB for a DVD ISO. This is roughly the size of a small to medium sized image file. It is highly unlikely that the Canonical servers will be overtaxed by downloads of these files.
A more likely problem is that the servers hosting the web pages containing the download links for the
.torrent files will be hard to access on high volume days. I don't see how adding magnet links could help with that. One would still have to access the (overtaxed) Canonical server hosting the web page which contained the magnet link to use the magnet link.