The motors are very similar. Some of the xr200's had dual carbs I would steer clear of those. I think the best way is to make a hybrid of the two motors. Tdup here on the boards has a bike built in such a fashion. I know for a fact that he runs alot of tight steep trails, so he used the 11 tooth front sprocket for lowend, and overcame it with the six speed where the topend was about the same as before. I don't know if he used any of the 200x tranny gears or not, it seems like the xr200 tranny would have way different gearing because of the larger dia. rear tire. The xr200 motors have alot of benefits, larger carb, higher compression (10:1) and the six speed, as well as probably a hotter cam. Adaptions would probably need to be made where the sidecase off the 200x could be used so that forward kick could still be functional. You will want to ask someone that has done the mod, I myself haven't tinkered with it, but it seems like a low buck approach to having a fairly hot 200x setup to me.
73 cr250m elsinore, 85 atc70, 85 250r flattracker, 87 250es bigred, 86 tecate, 2004 gas gas de300