Installing a four stroke in a X-factor Is relatively easy, You don't have to stretch or modify the chassis, It bolts in easily. BUT you have to change the gearing. I don't know what gears are available. My rough calculations, you would need a 45 or 46 tooth spur gear, and a 15 or 16 tooth clutch bell. Some people have asked about witch four stroke engine to use. O.S. made two engines a .26 fourstroke (compares to a .15 two stroke) and a .40 fourstroke (compares to a .21 two stroke) The four stroke buggy engines come with a pull start, and are painted black. But they are not available any more. If you watch ebay they do come up for sale some times. I have been asked if you can use an airplane version fourstroke, You can BUT you will be very disappointed!! The .40 fourstroke has 0.9 hp @ 17,500 rpm in the buggy version. The .40 airplane version has 0.65hp @ 12,000rpm. If you do want to use an airplane fourstroke, I would use a .70 or a .91, these would be comparable to what the factory X-Factor engine would have.