How can i fix my mountfield emperor mower drive?

How can i fix my mountfield emperor mower drive.
Mower works fine, but when the drive lever is lifted up, the mower stays still. The cable can be seen moving to and fro by the engine, so i know the cables ok, but no self drive.
This has happened previously, and i was told the pin had sheared. The original pin lasted 20 years, the replacement pins have been changed each summer.
I've resorted to pushing the mower now, so looking forward to a solution, apart from buying a new mower.
I have not moved house for the past 30 years