It could be very FAST with that engine so I would suggest propping it for constant power rather than flat out speed.

I run mine with a Magnum 61 4 stroke and a 13x6 prop, I've also run it with a Super tigre 51 with an 11x6 prop. The Super Tigre made it waaaay fast.

It doesn't need to be fast to perform well and pretty but that is just my opinion. With the 61rfs it will go verticle nearly 2 rolls worth and holds knifedge well, I think that the saito 72 is probably the perfect power source for it (or the 82 or a YS63 - dreaming...).

Enjoy it though, it's a great plane.