got to agree with fatblokeonbandit, if the mileage is correct and it hasn't had a real hard life then I would say the engine is ok, (they are pretty tough) the first thing I would of looked at was the carbs. they are getting on now and float valves are wearing out, ive seen this countless times, a new set of float valves usually fixes them. do a bench test on the carbs to rule them out before stripping the top end again.