Hi Pete, You need genuine Maico part #5020
All fitter and turners do spring-making as a part of their basic curriculum Pete, and this is a real easy one to jig-up.
I would try Les. Even if they were no longer available? I would imagine for sure, that he or Eric Cook form Maico only would have some one local who has made a batch for them.
Also Pete, I actually think an another way/improvement would be to fit an expansion spring instead, to "pull' the end of the tensioner up instead. You could attach the other end of the expansion spring to a thin bit of aluminum flat bar dropped down from between the lower RH air-box mount and it's frame attaching point.