Welcome to Outdoorking, Justo. I suggest you post some pictures of your mower, including a close-up of its serial number plate, in a new thread under Repairs and Maintenance/Questions on Lawn Mower Frames/Question on Scott Bonnar Cylinder Mowers. Be sure to post the long string of numbers stamped on the Briggs engine's cooling air cowl, so we will know when it was made.
Meanwhile, the black Briggs was a very good engine and unless it has deteriorated past the limits of low-cost repair, is a good one to keep. If you want to replace it, quite a range of new engines will fit directly onto your mower frame without modifying anything: the mounting geometry is standardised, though you do have to pay attention to the length and diameter of the output shaft. The reel can be straightened and sharpened as a fairly routine operation by a specialist reel sharpener, as long as there are only a couple of bends: if the whole thing is beaten up, that is a different story. New reels cost more than second hand Scott Bonnar mowers in good condition, so replacing the reel is not an attractive option.
