It appears that the central question here is can you use a "standard width hub."
Quadricycles and trikes don't have widespread or default hub standards in the same way bicycles do. There are a number of dimensionally different designs out there for how they attach to the axle, how wide they are, how they're driven, etc. In your case I can't tell whether the brake rotors are mounted to the axle or the hub, and you would also need to match that. Usually it's the case that like replacements can be had, but you need to take careful note of what you have to get the right thing.
If you're in touch with the company, they can probably tell you how to get 4 new hubs or at least what you're looking for. One catch is hole count - the stock photo above has 36h hubs. If that's what you have, getting the same hubs would then force you to figure out exactly what 36 hole fatbike rims you're going to pair them with, because 32 is the ubiquitous number for that. That's not to say there won't be any choices, but certainly far fewer.
It appears on this quad that the rear axle is always turning and probably has a hidden keyway to attach the hubs, and the rear hubs may not have their own bearings. (They don't need them in a design where the axle has its own bearings like this, but may have them anyway so that all 4 hubs or wheels can be the same).
You need to know all the dimensions for the axle keyway interface, the internal diameters for the bearings/bushing internal diameters that attach the hubs to their axles, and the spacing for those bearings/bushings (their width in terms of the first thing to touch the axle hardware to where the nuts on the ends press against). Then you can get hubs or have some made.
Depending on the hubs you can come up with and the rims you want to use, you may find a disparity in what spokes play nice with them. (The hubs pictured are using what are considered oversize spokes by bicycle standards, 12ga likely). You might be able to find 26" fatbike rims drilled for the big gauges, but whether they're the ones you want to commit to for a project like this is a separate question, since they'll be made for e-fatbikes and therefore will probably be garbage. There are 13ga nipples that work in normal bicycle rims so that's the "easy" direction to go in if you can find hubs that will take 13ga reasonably.
You'll have to think about whether in your redesign of the quad you want to stay with the original dish of the wheels or do something different, since it will be riding higher. If yours is like the photo above, you also need to figure out if there will be interference between the tire/wheel and the seats and if so what to do about it. It might be a problem that can be solved or helped by adding dish (making the rims offset toward their hubs' outer flanges), but doing that a lot also reduces total spoke tension and therefore also reduces strength.