Valve clearances should be checked around that mileage. Other than that, fresh oil and a filter is all the engine will need. The clutch should be fine.
28,000 is nothing to the engine... it should last 4 times that mileage with regular servicing.
I'd lubricate the suspension linkages, swingarm and steering head bearings, change the oil in the forks and service the brakes if these items haven't been done recently. I had an OEM front disc warp at about that mileage, not something you'd want to happen during a descent of an Alp!