as Clive said has it had a good service history ? has it been thrashed and abused ? saying that they are a strong motor. 60K is not high mileage for them. if its cheap enough buy it
can't say ive ever had a problem with a worn seat as a new valve fixes the issue. but ive heard of people using some sort of abrasive or cutting tool to clean up the edges of the hole in the seat. might be worth a try ?
are you sure the seats are worn ? the needle (float) valve has a rubber tip so generally its the tip that wears not the seat. have the carbs done a lot of miles ?