Hi Guys -
Thanks for the advice. I was able to get a few more spare parts and came up with a complete engine. Farmer John is right - the XR and XL heads are identical. I didn't measure the cam as of the 5 cams I have only one is any good (from the XL originally).
I was able to build a good running engine with the XR bottom end, another XR cylinder and matched Arais higher comp. piston, the XL head/cam etc, and the XL left sidecover (for the alternator etc.). The engine is in the XL frame for right now, with the XL carbs and a Supertrapp exhaust.
I'll eventually send the XL cylinder out for a rebore, buy a good 10.5 or 11:1 piston kit for it and have one of the performance cams I've got repaired. The cylinder from the XL is identical except bore size, so I'll end up with a 100mm bore and 80mm stroke for 628cc.
At that point it'll probably go back in the XR frame and see high country duty, but for now I'm going to enjoy having a hooligan street bike. I may even move the XR suspension over to the XL if it looks like it'll all work. My regular dirt bike is a '96 Yamaha WR250 and for the riding I do (40-60 mile days at Belfair, WA) it's perfect. But when the guys go to the high country (or Baja) I'll want the XR I'm sure.
Again - thanks for the help.
Michael