Personally I found life much easier (and spares more readily obtainable) with an Amal Mk1 Concentric fitted...
I also replaced the original gear set with one having standard ratios...The GB40 is very low geared for road use in my opinion...
I can't remember now if I changed the gearbox sprocket at the same time...
If the bike still has its 20" front rim investigate the availability of suitable front tyres...They were hard to find when I had mine but may actually be easier to obtain now...If not another rim would be a good idea if you tend to use it a lot...
Nothing else to do really unless previous abuse has occurred...They are a fairly sound and useable bike as they come...Ian