Redline started spacing the Monocog rear to 135mm in 2005. The 2004 and earlier Monocogs had 110 spacing. You could always reuse your stock hub if it's in good shape, other wise you have three options for a disc hub and umpteen options if you want to continue using v-brakes. SBS should be able to dig up an old stock hub, have your bike shop give them a call. Profile makes 110 spaced disc hubs and Kore also makes them. The Profile & Redline hubs are 32 hole & the Kore is 36 hole. If you plan on using BMX hubs, make sure that they're 3/8 axle.

I'm planning on rebuilding my Monocog too, so I've been doing research for a while. I hope this helps.