Remove the exhaust manifold and check in the exhaust ports on the head
If you see any which are wet looking (oily) then you can bet that its seen better days and is burning oil.
If its been sitting for 6 years youll need to pull it apart anyway as the compression rings will be stuck and unlikely they will free off so it will smoke like a chimney.
I would also completely remove the sump to check bearings and obvious marks on the crank.
But if its cheap say around £250-300 then would just buy it anyway (you cant lose) as its unlikely that every bit of it is fooked
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