Welcome to Outdoorking, Warwick. To talk about your Scott Bonnar usefully we need to know what model it is, and the best way to sort that out is for you to do two things: first, post a couple of pictures of it here (use the fifth icon from the left in the row of icons right above where you type your messages), and second, find the manufacturer's model and serial number plate on the side of the mower, and post that information or better, a picture of the plate.
The Kirby Tecumseh engine, also known as Tecumseh or Kirby-Lauson, was fitted to SB mowers and some others at the end of the 1960s until the early 1970s, so your mower will have been made in that period. It was an American engine normally made by Lauson, a subsidiary of Tecumseh, but the ones that are found in Australia were made in Sydney under a licence agreement by a local company called James N. Kirby, hence the Kirby on the engine identification plate.
