Bipes tend to have a lot drag because of the extra wing so they need more power than a normal monowing plane. I would recommend either the YS .63, the satio .72, or the OS .70 Surpass. With the first two being the most prefered. If this plane balances well with the .46 FX go with the Satio. If it needs more then look at the YS. I would use a 13x6 on these engines.

I think you would be surprised by how much wing area those two little stubby wings provide. Just add enough power to overcome the extra drag.