The replacement wheels will be flip-flop 29er wheels from bike island. Eventually, I would like to replace them with disc 29er wheels so I don't have to worry about this ever again. I seriously doubt I will ever generate enough force to shear six 4mm bolts.

As for the cog, I'm running it on my Monocog 29er now, and ran the same cog on a SS 26er before that. I bought an $8, 18t Shimanno BMX cog and drilled it. Just use a bit designed for drilling harder steel. Your hardware store should know what you're talking about.

And while I'm thinking about it ... I don't know of any 120mm disc hubs, but you could put a 100mm front disc hub on a 120mm axle with some spacers ... Its been done before. Might be less of a hassle and more reliable.

-Rob.