I've got an 888 one that will fit the early Monster.
Its got a gouge on the inside where one of the sprocket nuts came loose but it got me home from the middle of France with no problem. Its not pretty but can't see a 55bhp 600 stressing it as much as a laden 888!
You might need to file/machine down the chain adjusters to fit inside it though.
If you PM me your email address I'll try to dig it out of the loft and take some pics, or if you're passing through Alton one evening your wlecome to take a look.
11th year in DD #68 and getting slower by the year
M800 & 620SSie DD racer