I wouldn't bother putting a .60 in the PT-40. You can, but it's not really worth it, and the extra weight and messing around won't be worth the results, IMHO.

Many .60 size planes will fly on a .50, but not very well. So I wouldn't go that route either. (It does depend a lot on the engine of course. Most .50's are closer to .46's than to .60's in size and weight, with a bit more power. More of an upgrade from a .40 than a replacement for a .60)

In the long run, pick up a good .46. Personally, my favorate is the Evolution .46. The TT and OS .46's are also good. The OS .46LA is less expensive, but less powerful.