Armours will be the best place https://armourmotorproducts.co.uk/ Mine is a panel tank model but there was a different style for the later G3WO. You can mention my name if you want and they'll contact me for advice if required. (Military Ron)
They'll only supply them in bare steel or bright chrome or stainless. You either get your own plating or painting done or get the chrome and SS blasted to a dull finish. Ron
My only reservation is that the 1 3/4 WD system on their website shows a straight tailpipe. My friend bought his system from Armours and his has a straight tailpipe. My bike is also a panel tank G3WO.
They definitely make the curved tail pipe if they know that's what you want. I bought two or three last year for some Dutch members. Email them this picture of mine for confirmation. Ron