Most piggy back shocks won't fit in the frame but an older fox vanilla would be nice if you can find one. I fitted a DNM DV22 to mine which is fine and then i swapped the coil for something softer to suit my weight. You can pick one up off ebay etc just check the eye to eye legth. TBH you only need something basic, you don't need propedal as the idrive system eliminates pedal bob anyway, it's actually better without as you gain small bump response.
I have one of these. I've ridden a lot of single pivots and FSRs but the idrive system is just the best. The shock was bust when i got mine so i swapped it out for a coil over shock and it's just fantastic. With 35% sag it's small bump response is amazing yet at the same time it climbs like a goat.