I have a 18 hp briggs single cylinder that smokes terribly. I replaced the head gasket and it seemed to help slightly. Any suggestions of what might be the other reasons for excessive smoking

Did you do a good head and bore inspection when it was apart? Have you checked the oil to make sure it wasn't overfilled?

Do a compression test, then repeat the test after adding a table spoon of oil into the combustion chamber. If compression increased it could be bad rings