Hi Nigel,
you'll find most parts including some engine parts are applicable from the B20 right thru to the B26.
Don't get hung up on parts not being the correct number for your Bike.
You'll probably find its the same part with different number stamped on it .
I've have found this restoring my B20.
I wish there was a big database cross referencing parts numbers across the different models for what the same parts were used.
Good Luck
D