The mag mount in the picture looks quite small compared to the antenna. I'm guessing the mount is about 3" diameter, and the antenna is about an inch. I wouldn't expect that mount to work well if there were an adapter available. I wouldn't bother even looking for an adapter, and would just go with a different mount. I'd recommend at least a 4" mount, and if the antenna's more than three feet tall, perhaps even bigger, like a triple-magnet mount.
Here's a 4.5" magnet mount that claims to support antennas up to four feet long.
https://www.gigaparts.com/comet-antennas-cm-5m.html
Here's a triple-magnet mount:
https://www.gigaparts.com/mfj-336s.html
There are a lot of options, but I would recommend shopping around as mounts can vary greatly in price. You might want to avoid the cheapest ones, as they may have quality issues, but you could probably get by well with a middle of the road mount.