I always had a lot of fun on those XR's.. Personally I wouldnt put a whole bunch of cash into performance parts for it, an 87 anything has earned the right to be left alone.. Those old XR's are pretty easy to over ride the frames and boingers without mods..
Anyway, have the jug bored to the next size up, put in a oem oversized piston/ring/wristpin, replace the cam chain and guides, valves and valve seals.. Stick the new gaskets to her and bolt er back together..
If you do decide to go High-comp piston and such remember that reliabilty will go south and high octane fuel will be required..
On the other hand,, BBR makes all kind of really cool stuff for the 200..