Howard, engine choice depends upon you & what you like to do with the plane. The Seamaster will fly well with as little power as an OS .46 LA, & I have seen Seamasters with 70 size 4-strokes.

A pod-mounted engine has some disadvantages & one of them is significant mass above the aerodynamic center. More power doesn't always result in better flying (as an experienced modeller, you already know this). A good 46 size engine works beautifully with the Seamaster (TT .46 Pro, for example) and an OS 50SX will provide even more power without a penalty in stability that comes with excess weight.