Sounds like your current cam is RS (Australian for broken). Source a replacement or send it for a rebuild (personally I'd go for the wrecker option). Replace rocker(s) - poss wrecker again. Check compression and oil feed. I guess lack of oil started the rot. Once the cam was under oiled the rocker and cam lobe wore each other out and finally snapped the rocker.
If compression good and oil feed checks out OK replace the 't' chain, cam and rocker. Back on the road!
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i