What's the mileage?.. but on general overview lifespan of motorbike engine is 5 years before it needs to have some overhaul.. (either top or whole engine)..

An earlier batch of ER6n with their 650cc tend to be abused due to mindset of Malaysian think buying a superbike instead of street naked bike...

However, if you really keen bring the bike to a reputable workshop for mechanic to assess, or at least bring some of your friend who have knowledge on motors