Hi
Rick, and a warm welcome to the forum. Its great to have another Scott Bonnar Model 45 owner on board.
We all like to learn new things and tinker here.
To enable us to help you, we need some more pics of your machine from different angles, particularly the cylinder reel and inside the chain case, to assess the condition of the chains and sprockets.
Is that the original Briggs on the machine?....If so can you please post the complete Model, Type and Code numbers stamped into the metal air cowling that you have removed from the engine...They may be hard to see, but will be adjacent to where the carby intake manifold would be.You probably have painted over them.
These numbers will give us the manufacturing date of the engine.
We will also need a pic of the cracked clutch housing.
For now, I can tell you it is a Series 2, twin rail Scotty 45, made after 1975-early 1976 and is a 20" cut. If that is the original engine, with the numbers, we can give you a better idea of the approximate date.
