Well, the engine proportions alone would dictate that you multiply by 1.5, but...

I would probably go with a somewhat smaller ratio, say 1.25. Also, look around at commercial kits that exist in both .40 and .60 size, to get a feel for how some proven designs have been done. The details depend on what kind of performance you're seeking, and so you have to consider what construction details will change, and how the added weight will affect performance.