**Update** I spoke to my mate this morning who worked for Scott Bonnar....the engine was made by Scott Bonnar in Thebarton in Adelaide and was a very powerful engine, from memory he recalls that the piston is 2 1/2" in diameter and 155 cc....He and his mates raced that motor on a go-kart and won every race they entered...he still has the trophies....
But, sadly, he can't remember that black box or what is was for....
They were only fitted to SB mowers.
