There's a couple of ways to skin that cat, but IIRC, those wheels have powdercoat on them and will be touchy with rim brakes until the powder wears thru or you scrape it off yourself or you get a different wheel. Also, I can't be sure that those rims are even s'posed to have a braking surface on them, so please do be careful, whatever route you go, as there are some safety considerations.
As someone mentioned, you could get an 890-style caliper like they used on very old BMX bikes and still use on inexpensive bikes today. This would likely be the cheapest option, as all you'd need is a short-pull lever, the cable/housing, and the brake. (Keep in mind what I said above about rim brakes, too, though.)
A slightly more expensive/better braking performance option would be to buy an adaptor plate for V-brakes, which mounts to the bolt hole at the fork crown, and allows you to attach V-brakes (or cantilevers, if you prefer) to the front fork. For the v-brake conversion, you'd need the adaptor plate, a long pull brake lever, cable/housing, and a v-brake.
More expensive still would be to put a front drum on there. I don't know of a stock 29" drum front wheel on the market, and I don't know if you lace your own wheels yet, but if you have one made, it'll likely cost you more than the Genesis Onex.
From here, the prices for things go up dramatically. Theoretically, you could run discs, but that'd require a new fork, a new headset, a new stem, a disc caliper/rotor, lever, and a disc-compatible 29" wheel. That would probably run you at least $200 if you got the parts online and installed it yourself; it'd be far more to have a shop get the parts and install them for you.
If i were in your shoes, I'd ascertain whether or not that front wheel is safe to use with rim brakes, and then I'd go the v-brakes & adaptor plate route....