I had exactly that problem on my mint MC21SP, which I bought with 3600 miles on the clock, you can read about it here

Failed crank seal is not always a problem, but as Fontyyy said, many of these bikes do stand for a long time.
Anyway, splitting the crankcase is nowhere near as onerous as it seems, once you have done it a few times
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30