If your looking for an axle QRC Karts or Comet Karts offer different sizes...much easier than extending one, which raises alot of issues.
Comet Kart Sales - Axles, 40mm, 50mm, 1 1/4" Steel, Aluminum, Righetti Ridolphi, Margay, Top Kart, Shockwave, Azusa
Other than that touch up those...