I got a reconditioned airhead gearbox from James Sherlock for my R100PD Classic. James stripped it down and replaced every seal, bearing etc so that it was as good as new.
Kickstart gearboxes are hard to find although if you already have one they would do a rebuild.
Just to clarify, Rob means the gearbox that came with the bike when I bought it in Jersey. There was only one decent airhead mechanic in Jersey but he retired years ago. I wouldn't trust any of the other bike mechanics in Jersey to touch an airhead.
I sent that gearbox through the post to Rob courtesy of Parcel Farce and they managed to drop it on both sides and smash it to bits. When Rob tried to rebuild it it was beyond hope.
If you do send a gearbox through the post make sure it is very well protected and properly insured against damage.
NB the Sherlocks gearbox was perfect and super smooth. Worth giving him a call and asking what he has available. I think I paid just under £400 all in. This was for a replacement with no exchange.