Crankcase seal integrity is paramount for a 2T engine, once any of the seals start to fail, your only ever heading in one direction.

I've been down this road at the very start of my NSR ownership, it seemed a bit of a big job when I first realised the cases must be split, but its actually far easier than that.

You really do need to split the cases, and replace all seals which may be passing. Make sure you have a clutch drum holding tool, flywheel puller and engine frame / stand, this will make the job much simpler.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30