Mower keeps running ?

Mower won't stop

Hi
I have a mccullock 3540p lawn mower which has been well used and abused over the years never had any problems with it until now, when you let go of lever it no longer stops running I have had a little look and it seems that there is a little break shoe that is no longer applying enough pressure to stop the engine . Any advice how to fix this problem would be great
Many thanks

Ian