A set of pinions (4130 640 7301) is available from Stihl here and should be available Stihl on your side of the big pond. The last list I have was 71 usd. But not it is easy to get these FS gearbox part as you got to heat them to 140C in order to get the bearings out. per the service manual.
You may just want to find another used FS gearbox.
