Replace the piston first! (due to failure reports) Jet it correctly. If he has never been in the motor, it is probably jetted rich.
Buy a manual, read it, Every year do a compression check.... this is a good indications of condition. If you want a reference, Most KDX's can go several years without doing a top end. It just depends on how much riding you do. Usually if it starts easy (means good compression), runs good, has plenty of get up and go, you don't need a top end. Sure ...if you want to be anal about it as some are, you could do a topend every season... but it's not needed.
What kind of 2 stroke oil did the guy use? You will want to clean the kips when you replace the piston.