Can the focus head of Leica x typ 1 13 be replaced? Is it equivalent to 35mm or 50mm more suitable for travel and portrait?

It is irreplaceable. It's a fixed lens.

35MM is a small wide angle, 50MM is a standard lens, 35MM is more suitable for traveling, and 50MM is more suitable for portrait.

Let's use an example to explain. If your family travels to the West Lake, when passing through the broken bridge, the width of the bridge deck is 8.6 meters, and your family stands by the railing beside the bridge (assuming that your family is 1.8 meters tall and your children are 1 meter tall), with a 35MM lens, you can stand almost 4 to 5 meters, that is, standing in the middle of the bridge can take pictures of your family. If you use a 50-lens camera, you have to stand almost 7 meters away, that is, you have to run to the other side of the bridge to take a full-body photo of your family. It can be seen that if the 50MM lens is inside some buildings (compared with visiting temples), it is basically impossible to take a full-length photo, because there is no room of 8 meters inside the temple for you.