I believe the stock length of the spindle is dependent on whether you have the Titanium or Steel pedal version to begin with (at least this is the case for the X series). I think the stock Ti spindle is 50mm, and SS is 53mm.
Here's one (rare) place that seems to sell all of the options for Zeros; these seem to be official Speedplay versions.
https://www.i-ride.co.uk/products/co...ku=03024100607
I don't believe Speedplay sells any spindles longer than the stock size that are made out of titanium. OTOH, it seems to be a lot easier to find aftermarket spindles on Ebay that are made of Ti than it is to find them out of stainless; not sure why.