Hi Humdinger, and a warm welcome to the forum. Its great to have another Scott Bonnar owner on board. grin
The SB Model 33 was a 16" cut machine, first introduced in 1960 and manufactured at the Thebarton factory. It ceased production in 1966.
It was manufactured in 2 versions....the first was with a Villiers 98cc. Mark 3 'Midget' 2-stroke engine and all chain drive. The second was with a Kirby-Tecumseh HK 25 2.5HP 4-stroke engine and was a hybrid drive of 'V' belt and chains.
I have not heard of a Model 33 being factory fitted with a 3.5HP Briggs and Stratton; as that engine was fitted to a much later Scott Bonnar Model 45.
I would very much like to see some pics of your machine, with the chain case (side) cover removed and of the ID plate affixed to the mower.
Some parts of your machine will be rare and hard to source. wink
Once again Humdinger, :welcome: to OutdoorKing.
cheers2

Please do not PM me asking for support. Please post your questions in the appropriate forums, as the replies it may receive may help all members, not just the individual member.
Kindest Regards, Darryl grin