That is quite odd. Probably done because a continuous belt is more expensive than a cut to size one.
Unless you’re adding accessories that will add weight (auto focus, laser pointer), the mount is attached to the linear bearing block, so it should be fine. If you’ve got slop now, you may need to replace the bearing or belt. Something else you could do is fabricate a longer mount and add a second linear bearing block. I’ve seen quite a few plasma tables with side mounted linear rails that support the steel gantry, the weight of a laser head shouldn’t be an issue. Check your bearing block for a small hole on either side for lubrication, sometimes it has a set screw in it, go to a Hobby Town or similar store and get an RC car lube pen that has oil in it and a needle for just that kind of thing. Just putting Super Lube on your rail isn’t enough.
For the replacement you asked about:
Two options, rotate and rebuild your gantry to match the vertical design (more expensive if your pulley and motor mounts won’t reconfigure and have to be replaced, plus a new belt), or buy the new head and use the existing horizontal mounting hardware (looks like you’ll need to drill and tap the mirror mount if the replacement head tube won’t just swap in). I have two lasers with different size lenses and I wanted them to be the same. I bought just a new tube. Since your laser is 100W, I would suggest getting the tube with the additional option to mount a 4 inch lens if you want to cut thicker materials.