They are generally reliable engines, theres a handful on the forum with over 200,000km going strong. Valves more often than not to tend to be in spec, or stay in spec after an initial adjustment. They also use little to no oil. More likely are tired suspension, sticky brakes through lack of cleaning, abs units can cause trouble if the fluid isn't changed regularly. Also check for damage from low or no speed drops, a high c of g and a lot of weight can catch you out.
