Briggs & Stratton 18hp engine bogs down and blows white smoke when put under a load. Just replaced the needle valve because there was a lot of oil in the gas and engine would not start. Could the excess gas in the oil damage the rings or valves?
Thans in advance for any advice.

Briggs & Stratton 18hp engine bogs down and blows white smoke when put under a load. Just replaced the needle valve because there was a lot of oil in the gas and engine would not start. Could the excess gas in the oil damage the rings or valves?
Thans in advance for any advice.

I assume the oil in the gas is a typo... in the first highlighted portion because how would the oil get in the gas... but the second highlighted portion can result in everything up to complete engine failure due to poor lubrication. And the smoke may be an indication of exactly that, excessive wear.

Why would you put oil in the gas at all or did you mean oil was seeping into the cylinder?

You should change your oil and filter ,
After you put in new oil ,it might take a while to burn the cylinder out and the muffler , 'I have had them smoke for as long as 45 - 60 minutes.
Luck

The oil in the gas was a typo. I was getting gas in the oil due to a faulty needle valve. I have changed the oil and filter. The white smoke has stopped after about 20 minutes of running, but the engine still loses power when I engage the deck.

Does it just pull down or die ?
You might check your valve clearance they could have strecthed, you might check and make sure its running at proper Rpms at high speed 3200-3400
Luck