I think the 89 has an alloy cover so no cover rotting away ( fairly certain). The things are bullet proof. No issues, plenty of parts. Ebay is your friend. Have fun!
Plastics.
http://www.maier-mfg.com/catalog/cr125250500-side-0
Yeah,
After some further investigation, it seems to be rock/stick damage, as opposed to corrosion.
When I was first looking at it, I checked the rads and noticed they where empty, I queried this and the seller said they had devconed the waterpump area, so I instantly assumed corrosion, but it appears damage has been done to the pipes that exit the top of the pump that the rad hoses attach to. The gear oil is clean, no custard there which is a relief.
Frame is freshly painted, and looks to be a half decent job, and topend is brand new.
Besides the waterpump, the only other bits it needs are Fork and shock rebuild and linkage kit. New wheel bearings, brake pads and a rear rim, some new plastics and away we go.