A 55W or a 40W PL-L lamp would be a pretty good fit for a SOX 90 sixed fixture. As far as converting it, it's fairly simple. Just find some screw holes in the fixture that match the holes in the ballast (or make your own), the same goes for the lamp holder. Fit them in and connect them. Be mindful that some ballasts can't have long wires running to the lamp, but it should say on them if that is the case. You will still need a lamp support for the PL lamp, the SOX one may work if the lamp rests down on it, or you may need to fabricate a new one. Here is one I did, converting a 35W SOX lantern to 24W PL-L. It's not the neatest job ever, but it looks decent.

Interesting, although I would have guessed that 20 tonne thing Kev has got from DW Windsor?, with its “slow ramp up diodes” controlled by circuitry like something off the star ship Enterprise! 

I still want to get a SOX streetlight (Palomar by Spaulding) but due to the lack of ability of lamps, I unfortunately would have a hard time getting a SOX lamp that cost less than my fixture.
As such I plan on replacing the magnetic SOX ballast with an electronic programmed start fluorescent ballast, and the BY22D lampholder with a 2G11 one, and instead of using a SOX lamp, I would use a PL-L lamp.
Does anybody know what size of PL-L lamp would be appropriate for a 90w SOX streetlight and how I should go about doing this?

Someone sent me this google street image a while back saying they're somewhere in Greenland. Can anyone help identify these fixtures?

At EOL SOX lamps were never recycled. The lamp was hard to make alright but from easily accessible materials.

Sounds like a waste of resources but okay...

SOX lamps are the one kind of discharge lamp that can be disposed of with regular household waste. What you are supposed to do is put the lamp underwater and break it so that the sodium can react with the water, therefore neutralizing it so that there isn't a risk of it causing a trash fire from coming into contact with water.
Once that has happened you just put the shards of glass in the trash and you are good to go.

What happens to the various parts of a SOX lamp after it has been disposed of?

This is as difficult to answer as to name the inventor of the electric lamp.  That title is usually given jointly to Joseph Swan and Thomas Edison, but in fact working electric lamps were made by dozens of other individuals many decades before them.  Edison & Swan only achieved fame because they were the first to produce a commercially viable electric lamp - one that was affordable, bright enough and with a long enough life to meet consumer requirements.

The same is true of LED streetlights.  There were experiments dating back to at least the 1970s but their light output was pathetic and the cost was huge.  It took another three decades before LED streetlights became both functional and affordable.

I believe the credit for the worlds’ first commercially viable LED streetlights goes to the Belgian company Schréder, which in 2007 made perhaps the first effective and large-scale installation at the city of Split in Croatia/Hrvatska.  Just prior to that in 2006, Cree demonstrated advanced prototypes of high power white LED streetlamps that had been made possible by the invention of their Xlamp 7090 emitters, but I don’t think those were sold.

I'd probably say Leotek comes first. RUUD Lighting/BetaLED comes in second. There was a Leotek SL-36W1R-FX in my town that's probably been there before 2007 because it still shows up on the 2007 street view here.

Leotek even made LED streetlights as early as 2004!