Rong, it seems to me you've established that your mower has the wrong engine and is the wrong colour, but other than that, it does not seem clear that it has any modifications. Bob's parts list does show smaller tyres, but it is not certain that the parts list applies to your exact model. If they don't foul on things and limit your turning circle, usually oversize tyres have some advantages. They make it more practical to operate on slightly soft ground for instance, and improve both traction and ride comfort.
