displacement is basically the volume of space a piston travels.
stroke is how far the piston travels from the bottom if its stroke to the top.
bore is the diameter of the piston.
2.5hp @ 34,000 rpm means that the engine has 2.5 horsepower at 34,000 rpm. thats probably that engines peak horsepower. less or more than 34,000 rpm and that motor wont have as much horsepower