Any news on your research? I'm actively looking for a suspension replacement myself, but it seems that even OEM Sierra/ZS 200 GY-2 rear shocks are unobtainium, and I have no definitive measurements/numbers to go by.
Edit: I contacted the OEM source I mentioned a few weeks ago, but it turns out they have ran out of stock, selling the last one about a month ago
Seems they were quite in demand...
Sadly, from a quick search, neither aliexpress or the UK-based
China Parts LTD has a compatible shock anymore (the generic "GY 200" one has different mounting sizes top & bottom). Worse, the guy who ran out of stock also told me that these shocks are not rebuildable
I wonder if the only upgrade path is to fit a different shock mounting bracket, a leverage system, a Yamaha rear swingarm or weld the old mounts on a new shock...
Edit: By comparing a pic of the stock shock:
and the standard Yamaha TW200 shock:
they do look very similar...in particular the differently-sized mounting points. I wonder....