G'day Mate,
They recommend a 40 to70, the 40 is a 2 stroke the 70 is a 4 stroke, which is about equivelent to a 46 2 stroke, it happens to be in the group 40 to 52, 2 stroke.
You could use a Saito 82 4 stroke, or an OS 46AX, or a 70 4 stroke, whatever your budget runs to.
The Saitos are excellent engines, but expensive.