The 83 is slightly different (the radiator inlet points inwards compared to the 81 & 82 & one of the main mounting points is slightly different).
Also the 81 is alloy wheres the 82 and 83 are mag (which rots out).
I got an 81 cover fitted onto my 83 CR250 and just got a custom pipe made to fit around the inlet. Also got a guru mechanic (Anthony at Powerhouse in Melb) to fit it up ot the case.
The 83 has many parts which are unique, which makes the parts hunt challenging but rewarding!