I dont' know if you can get master cylinder sight glasses. I do know the clutch side cover for a bw200 has a seperate sight glass and gasket though. I don't know if it will work for you, but you might want to look in that direction, for side covers with sight glasses in them.
Nicholson 500x (Project)
Nicholson 185s
Nicholson 110 (That takes an atc70 tank)
Ascott 500 head/350x (Project)
Homebuilt Racer Chromoly Suspension 110 (Ragin Runt)
PK Racing Suspension 70
A+ Inc Suspension 70
AWS Aluminum 90 frame
Hi Performance ATC Suspension frame rd350
Hi Performance ATC Suspension frame 90