Welcome to the Huni Rex club 
I did a write-up a while back on mine.
I’m fairly confident that it can fit a 29" wheel without requiring any modifications to the frame. I was considering getting another and building a 29" Huni as well. From what I know about the hub, it’s a proprietary deal; as far as I know there aren’t any other hubs that’ll fit in a Huni Rex (though if there’s anybody that has tried a different hub and made it work… I’d really like to know 
).
The way I would do it is to unlace the current wheel and reuse the hub for the build. There is a catch to be aware of: the cogs on the hub are fastened using lockrings and a retaining compound. I highlight that because if you need to take off the cogs for a wheel build, you’ll need to reapply the compound so the cog doesn’t shift back and forth under load and chew up the splines on the hub (ask me how I know 
).
The retaining compound + catalyst is a little spendy, and to do the job right will require some patience cleaning off the old compound. But it’s entirely doable if you’ve got the time for it. I would also recommend you get the cogs off before unlacing the wheel since you’ll need some leverage to get the lock rings off (one of them is right hand threads, the other is left handed threads).
For the brake, you might be able to get long reach v-brakes to work with a 29" wheel on the upper pair of holes.