you might have to make your own, like this:
New Complete Jackshaft kit, 5/8" by 10". This is very popular for use on Go-Karts and Mini Bikes. This Kit includes 5/8" by 10" Jackshaft With 3/16" Key way. 10 Tooth Sprocket with built in Key for #35 Chain.
www.ebay.com
However, you'd need a 20t-24t sprocket, instead of the 14t.
You'll also need a centrifugal clutch. If you get a 10t clutch, get the 20t sprocket.
If your new clutch has 12t, get the 24t sprocket.
However, if you're use a bigger chain than #35 on your rear wheel, you'd need to get a different output jackshaft sprocket.
So you might end up needing to change both sprockets.
It might be wiser to engineer your own jackshaft, piece by piece,.
You can get all the parts on Ebay or go kart companies.
Sounds complicated, but it"s just tedious.
Watch out for shipping charges. They make things more expensive.
Try to buy everything from the same vendor. They might give you a break.
Good luck.