Hi again jaredr,
Do a search on this site for KDX250SR and you'll find some more info that has been posted about this bike.
Your questions...
Is it safe to say I can use plastic off of a '94 KX250? No, plastic from a 1994 KX250 will not fit. All plastic from a 1991-1994 KDX250 (US version) will fit. Typically KDX and KX plastics (of any year) do not interchange.
Can I rebuild the bottom end even though I don't know how to read Japanese (I've heard the engines are different, but how different)? I haven't compared bottom end part numbers to determine if they are interchangeable (between a US version and an SR model). The piston is definitely different - note my previous post. Power valve system is also different - although the side vavles are the same ones used in the US version.
You can use many of the parts from the local Kawasaki dealer to fix up your SR. My dealer said he could not get the SR specific parts even though I had a good Kawasaki number!
You can email me with more questions if you like...
jagkdx@yahoo.com
By the way, I have a 1992 KDX250SR here in Manila, and a 1993 KDX250 (US version) in Pennsylvania. So I have a bit of experience with both bikes. I actually took a seat from the US version to Manila for my SR... it is much more suited for American sized people! The original SR seat looked like a ski-jump... the profile was definitely for a shorter rider market!
Jim