Always follow the main rule - When you have bike issues always look at the cheapest and easiest parts to replace first.
The seals that can cause this are the crank seals. Normally there is one on each side of the crank. The right hand seal is normally immersed/semi immersed in engine oil so tends to last longer. The left hand seal, when it fails, lets air into the combustion area making the bike run lean and increases the risk of a seizure. The seals harden over time and stop doing their job. Bad idling can be down to the left hand crank seal. The problem can be worse when the engine is warm.
The right hand seal change is normally only possible with an engine split. The seal would let in engine oil and this is burnt causing a more smoky 2 stroke and the bike will use more engine oil.
The left hand seal can normally be removed with the engine in the bike. This is normally done by removing the left hand crank case cover, Removal of the stator, which exposes the seal. The seal can be popped out and a new one put in.
A check for the left hand seal is to remove the left hand crank case, start the engine and to spray WD40 behind the stator plate. If this is leaking then the engine note changes. This sometimes doesn't work but often does.
This is a general guide and you would need to check you bikes set up before doing this.
In my TS250 thread - which is in the project section - shows me doing this to the TS. It is towards the end of that thread.