The conversion is not easy. It's actually easier to throw the 200 engine in a 125 chassis - but that is still $$ and takes lots of skill.
Then the engine is wimpy compared to what the chassis can do.
Want to make the XR feel like a CR? Plan on spending at least $4000 to do it (including some engine mods). This is IF you can do the welding/fab work yourself.
Now, back to the real world - here's places to get bolt on's etc for the XR200:
Graphics - N-style still makes nice looking ones. You can also use the XR250 stuff - cut it down.
Front Fender - Acerbis universal
Tank - Nothing available aftermarket
Rear Fender - Nothing available aftermarket
Ceet - can make a custom seat cover for the XR200 in any of their designs (non slip, gripper, etc) for around $140.
Works - Real rear shock - set ya back $400 to $600. It makes a difference. Still not as good as an XR250 though.
Progressive - Heavier fork springs - $80.00. A definite gain and good bang for the buck.
Rear Excel Rim & relace - relace to fit a REAL 18" rear tire. Cost about $400 with new tire - less if you purchase the spokes and do your own relacing work.
Powroll 218cc stroker engine - No other engine mod you can do to the 200 will compare. Cost $500 to $1,200 depending on how much you want to do and what shape the engine's in.
I've owned a bike set up similarly to the list above. I'm in the industry, so it didn't cost me nearly as much as the retail prices listed above. If I had to pay retail for the stuff, I would have NEVER done it.
Bottom line? Unless you're really short, or want the ultimate woods play bike, buy something else.
Want pictures? There's a few here
www.powroll.com it's on their XR200 page.
Hope this helps you out.