Look at this parts list illustration again:
![[Linked Image]](https://www.outdoorking-forum.com.au/forum/uploads/usergals/2014/12/full-2772-18895-rover_rancher_18166_pulley_system.jpg)
On Item 10, the Arm - Jockey, I've circled in red the place where the spring, Item 34, attaches. I've circled Item 34 in green: it is called Spring, Jockey Pulley. The spring's Rover part number is A07974. I don't know whether they still stock the part, but considering how many models that deck has been used on, I expect they do. This should be considerably easier than trying to invent an equivalent spring.