I have had thoughts of this as well over the years - and also a turboed
CR500 based twin.
Anyways...
I reckon a turbo would be better in a motard a you dont have to worry about an airbox as much and can shove the turbo in that space instead. Easiest would be one of the newer models that already has injection, so already has fuelpump setup etc. A number of them already have the ability to be reprogrammed, though to what degree I do not know. Also, I know Honda was having issues with response on the CRF250, it never ran as well as the carburetted version initially, feeling very flat. No idea how the release version goes. The 450's don't seem to have that issue with injection.