Any luck Andrew?
I’m not sure what the back-end mounting points look like on a ’06 First Kart, but most will use the bolt/bushing kit here:
http://www.fastech-racing.com/italkart-rear-bumper-bolt-kit-new-style.html
Last year I updated a 2005 tonykart with full width rear bumper. I recommend the quick disconnect mounting kit, though it does weight a tad more than the regular RLV kit:
http://www.fastech-racing.com/righetti-ridolfi-mounting-kit-for-xtr14-rear-bumper.html
Also, I was originally going to get the KG bumper (I think??) and Fast-tech told me they are too soft. I went with their recommendation and got the Righetti instead. It’s held up nicely.
http://www.fastech-racing.com/righetti-ridolfi-xtr14-rear-bumper-black.html
I usually deal with Fast-tech or Acceleration karting for parts; they are very good/friendly, but I’m sure you can find these at your local shop if you prefer to support them.