My friend bought a 2007 r6s from a dealership. It looks like a wanna be drag racer bike. Stretched, lowered, rear brakes taken off. So anyways, I was doing a service on the bike and i noticed that the previous owner used the netting from a sliding door as a air filter. Inside the cylinders are dirty. So my question is how can I clean it out. Or should he just clean whatever he can and sell the bike? Bike got 9k miles. I dont know how long its been running like this. This is going to be a headache.

Use those clorox wipes and clean everything you can.
Take a compression test.
Start the bike and if it puffs smoke = Sell