I am pretty sure the petrol tank is different the way it fits in place at the front, the R5K has a bar which locks under the frame where as the R2J does not, shouldnt be too hard to modify with a hacksaw though .

As to the bodywork, I am pretty sure it fits straight on without any probs. I have the R2J bodywork on my R5K and it fitted straight on....the bike had no plastics when I bought it and I got a full set for it but did not know the difference at the time.