i single speeded my old rockhopper. the surly tensioner works fine although you can probably get a knock off these days for a bit cheaper. once you have that, just get a cheap BMX cog and a spacer kit for your freehub body and you're good to go.
if you want to do it on the cheap you can just use your old derailleur as a tensioner. kind of cludgy but it works. i miss my SS MTB, wish i hadn't sold it. now i'm lusting after a karate monkey for some hot spring dirt action.

BTW - i have an extra set of Time ATACs. kind of a shot in the dark, but would anyone want to trade these for a pair of (used of course) egg beaters?