There is a dvd available now with the px engine rebuild it is very good, for £22. Also this months Scootering has a very good article on px exhausts.
The cylinder, piston and carburettor jobs are very easy, you dont have to take the engine out of the frame, and the cables oil line and petrol line on the carb are very simple. Take loads of pictures ect. as a reference for the rebuild and also use loads of gasket paste around the cylinder barrel to engine and cylinder head to barrel, should help with any leaks ect.
Gary




