I've got a few recommendations for modding your levels and/or getting the feel right. First, your PCs are going to know that kobolds are weaklings so however your math shakes out, I would recommend most fights have at least 2 monsters for every 1 PC. This will maintain the feel that kobolds are low level, since the PCs can mow a lot of them down.
Next, if you're willing to do the work, applying elite templates is equivalent (in terms of XP) to adding 4 levels to a monster. IE: if you have a level 3 or 4 kobold and apply an elite template, he will suck up as much XP as a level 7 or 8. For this you will have to be careful to make sure these guys can still hit your PCs.
Minions work in a similar concept. minions are 1/4th the XP of the same level normal and every 4 levels XP doubles, so as people havem mentioned a level 1 kobold is worth the same XP as a level 9 minion.
If you're feeling a bit lazier than that, but still willing to do some modding, while the original DMG recommends not modifying by more than 7 levels, the new math works out that for every level you shave off/add on 1 to hit, AC, NADs, and damage (previously damage was 1/2 but it is now just 1 like everything else). Then do 10 HP per level or look up the specifics if you want (but you don't have to). Chris perkins seems to be comfortable modding things up and down as much as 10 levels and letting it ride. It's going to be a bit off, but it does work. a week aggo I modded an elder blue dragon, lvl 20, down to level 14 to challenge a party of lvl 11. It was very swingy and the party nearly died, but they also finished him off without using up all their dailies. Long story short, it works.
If you're feeling even lazier than that: as others have said, a kobold is anything that's size small and shifts as a minor action. You can literally take any monster, strip off existing racial powers, and do this and it works.
Finally, if you're still trying to preserve the flavor of kobolds, I used this trick to great effect: swarms. The name escapes me, but there is a swarm of floating heads from open grave or something. If you envision the individual heads as kobolds the powers all make a decent amount of sense. Increase the size of the swarm to huge and it's like the PCs are fighting an entire army of kobolds.
Hope this helps.