dosnt seem to be any oem or replacement sight gauges around but someone said a Kubota mini tractor sight works or a plain black rubber hole cover.
in google search for the following
kubota hydraulic oil sight glass
Kubota Hydraulic/Transmission Oil Sight Glass (Guage)
and look in images, loads of options come up, guess you just need to work out the thread size and order something to fit.
anyone know the thread type and size of the original sight glass for bart?