Some have done away with the fixed position idler altogether and replaced it with:

1. A complete rear derailleur cage.

2. IF those are square taper BB's, pick up one of the eccentrics the fixie crowd is using to convert vertical dropout road bikes to fixies. They won't take up much slack, so size your chain to be real close. Might have to use a half link.

3. A ghost or floating ring. Get the chain real close and insert a bigger used chainring in between the two timing rings. It stays put, takes up the slack, and has people going "What the ...!"

Of these, #3 is the easiest, fastest, cheapest, quietest, coolest, and several other -ests I can't think of right now. But to each their own. If you find one you like, you can cut off the annoying idler arm bracket, file down the rest, and grab some white fingernail polish. A man on bicycle-back would never know it was there.