Small Engines (Lawn Mowers, etc.): Riding lawnmower, farely, dramatic loss

Question
My cousin has two riding lawn mowers that experience a dramatic loss of power after running for about an half-hour . The power loss is so great that the lawnmowers are unable to finish cutting the lawn and have to shutoff.My grandfather use to tinker with these lawnmowers to occupy his time. So, I am wondering if he messed something up. One of the mowers is older , but the other is farely new (2yrs old). The newer mower it turned out had some debris in the gas tank , but this was removed and a new gas filter was put on. However, the same problem still persists.Could gas with debris have caused permanent damage to both mowers?What are you thoughts on the situation? The newer mower is a Statesman. I am unsure of the engine ( between 12-16) , and probably B&S .  

Answer
Have you replaced the spark plugs?  You mentioned debris in the tank; have you cleaned the carburetor?

Are the engines clean?  Have they been degreased and are the cooling fins and flywheel fan clean?  You will have to remove the engine shroud, top engine cover, to check the cooling fins and flywheel.

Let me know what you find.
Eric