Gday Vic
Has the master cylinder piston got a hole for a push rod to locate in or just flat ?
The old BMW twins with rear discs used a similar master cylinder that was horizontal mounted with a remote reservoir.
Like this
http://www.ebay.co.uk/itm/1979-BMW-R100 ... 0594407684
It had a short lever that pivoted on a pin that went through the hole near the 81 batch code on your cylinder it converted it to pull on instead of push on.
Is your set up pull on or push on ?
If it is pull on see if you can locate an old BMW one and use the arm/lever.
I had a look on the real OEM site at the BMW part but you cant get the arm/lever as a separate part for some reason.
I have to say that the BMW's rear brake was dreadful using that set up.
Good luck with your quest mate
Roger