MZ/MuZ Riders

I need to replace the final drive socket on my es250/2. It is currently fitted with a 19tooth sprocket..the manual says it should be a 21.
Should I get a 21? The bike is going to be ridden solo.
Does anyone know where I can get one...I'm based in the UK.
MNy thanks

The only place I can think of at this present time is,

http://www.mzspares.com/contact.htm

As Fred Rodgers is out of action due to ongoing health problems.