Many thanks in advance for your input

I have a Briggs & Stratton 18.5HP model 31P777-0296-E1 engine in a Craftsman riding mower, I bought the mower in 2005. With regular maintenance the mower has performed flawlessly until now. The mower recently started exhibiting the following problems.
1. A oil or oil/gas mix drips out of the fuel pump vent when the the engine is running.
2. There is an excess amount of exhaust smoke during operation.
3. The engine runs fine with the above problems for a while and then start to run poorly, (runs rough and loses power).
To date I have replaced the fuel pump, had the carb rebuilt (although I have may doubts if anything was actually done), changed the fuel filter and air filter.
Any ideas would be greatly appreciated!!