Load the skybolt. Open the Airplane editor (Airplane->Edit Skybolt). On the list in the lower-left of the editor, expand the "Airframe" item, then in the list that appears under Airframe, expand the "Fuselage" item, then in the list under "Fuselage," click on "Engine." Now on the right hand side of the editor, scroll down and look for "Torque Generator." That shows the current engine. You can change it by clicking on the "..." button to the right of the engine's name. Pressing it will bring up a list of engines, one of which is the "OS FS-1.20 SURPASS SP." Close the editor, choose a name for your new aircraft, and you're done.