I think you're right that the MC18R2J might in fact have a 4" rear wheel. I was thinking about that this afternoon. I can't find anyone in the UK selling 140 rear tyres though. Not new ones anyway.

I only have a HRC F3 R2J to look at and that has a 4.5" rear wheel the same as the R5K. You could however swap your R2J rear wheel for an R5K one if you wanted the extra 0.5".
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If I have to take the carbs off once more...