Replacing a seal requires the crankshaft to be removed so if you don't know how much work its done replace it as well includung the main bearings, costly but you will $$ in the long run. If the piston is really carboned up then there is a good chance the kips are as well so give them a clean. I use a wire brush on angle grider to get them spotless. The piston (top only) I clean it with metal polish/cleaner. Rips the carbon right off and shines it up real good. If the power valve is hitting the piston it will leave big scratches on the piston and can also hear it when idling. Costing at a shop needs you to go to them and negotiate a price. I ordered the crank/bearings/seals and stripped the motor to the stage of splitting the motor let the shop do the rest. Cost me Aus$100 labour on top of the cranks etc. There are other costs like new piston ring, gaskets, seals etc.
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