Engine kits all comes stock 49cc 2 stroke and 80cc 2 strokes with a 44 tooth sproket, Except for thatsdax with a (41) tooth sproket, I have a 44 tooth with an 80cc slant head engine, My top speed is 34 mph (doesn't take long to get there =), However it JUST Has enough torque for a fairly good hill, The larger sproket you get (More teeth) The more torque you will have, But you will also lower your top end (Top speed) With the larger sproket you get, The 49cc pretty much has the same top speed as the 80cc, But the 80cc has more torque. i'd got with an 80cc. See how the 44 tooth takes the hills, and if that doesn't work out it's only 30$ for a 50 tooth or so