check the passenger side seat lever - dont know why but they are ALWAYS missing
on a more serious note - tappets will always be noisy on the 899cc, if you fix them they only start being noisy again in about 3 months (if that)
check for overheating, should be sitting at around 70 mebe 80 degrees after its warmed up and driving around
check for oil leaks - erm, what else went wrong, oh yeah ask about the fuel tank and whether it was replaced or not.
all the usual things when testing a car, look on black seals to see if the car has had been resprayed and theres overspray - sign of an crash or rust. check the wheel arches they can start rusting.
there will most likely be a spot of rust on the bottem right side of the windscreen (underneath the windscreen not the glas itself!!)
check tyres, check service history, ring a previous owner to ask about the car if possible
take it on a really good test drive