Hi there
I am not convinced its big ends, It doesn't sound like 2000 rpm knock to me, It sounds more like transmission, first thing I would check is flywheel bolt then check the clutch hub nut holding clutch basket to main shaft there is a spring washer and a tab nut to dent in maybe its come undone

I spoke with Arron tonight and he said he will reimburse you and collect it if you want to sell it,
Good luck,
jonmac
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John