W116 rear sway bar
W116 rear sway bar
My car was missing its left rear link when I bought it a month ago, this will seem ridiculous, but I "fabbed" up a new one with zipties. Its been doing its job for about a month now. I'm really surprised it hasnt melted from brake heat(braking to the point of serious fade on a few occasions) but so far so good. Might be worthwhile as a temporary fix in your case.
As far as larger rear bars from other w116's or retrofitting larger bars from other models, I havent heard/read of much success. Seems like the larger bars were only fitted to w116 ambulances, hearses and such, unicorns in america. I haven't looked into it much yet, but im hoping theres a reasonable/easy way to double up 2 stock bars. I do agree that a stiffer rear bar should help the understeer situation, these beasts are far too fun sideways haha.