I need a 50mm axle but the longest one I can find is 1040mm, roughly 40 inches long.

I'm confused....
Do you mean 50 inches??
A perfect example why america's ignorance to the metric system
is a baaaad idea..
innocent kids are likely to get bullied from all around the world for making an utterly stupid comment like that!
bob, I'll see what I can find..
I think I saw a race-kart style sbs two seater once for some track day sightseeing..
I think it was 20-25cm wider..
'sid
[
EDIT]

So I checked the two karts I had in mind (Sodi and Rimo)
Sodi 2Drive is actually just 1450 mm wide... so that'd be a std length axle I'm afraid
the Rimo Twin is 1530 wide, wider but by far not as wide as I thought it was ...
frankly, I doubt it has a wider axle with just the 80mm excess in width..
more likely they included the bumpers that sodi likely ommitted in measurements
maybe extended hubs *shrugs*
What size would you need and what keying is required?
since while it's easy to just cut some tube to size,
that wouldn't still make it a go kart axle..
but if you need a special size, maybe contacting righetti ridolfi could solce your problem,
they make kart axles on a daily basis maybe they can cut one oversize one for you and add the requied keyslots'n'holes