Yes Peter,
The lack of a decent brake on Greenfields has resulted in people being killed so you have to be very careful. The Greenies that were fitted with a gearbox were a real problem on hills if you had to change from forward to reverse, they can get away from you very quickly. I have fitted a hydraulic brake to one of them for this reason..
The chain tensioner is similar but no the same as the Anniversary models so if you can't find one you can make one up easily with a bit of flat bar. You don't need to make the adjuster part just set it manually and check the chain tension every so often
