Some helos can be easily modified to accept a larger motor, some can't. Modern .46 and .50 sized helos are based on .30 size helos. That said I had a Century Pro .30 that I ran a OS .46 in. the only thing that I needed to put the motor in was a engine mount from the Century Falcon. The real worry about using different engines is their operating RPM, .46 and .50 engines turn slower than .32 and .34 size. The pinion gear from the Falcon was used to bring the head speed up to normal. What this did was take a great flying cheap helo that needed a high dollar pipe and 30% fuel with a .32 into a screamer with a cheap muffler and 15% fuel with the .46. The .46 and .32 OS were about the same price and the pinion and motor mount cost less than the tuned pipe for the .32. Going any bigger would be a waste of motor and would be very difficult.

Another case in point is the Freya I now have, designed for a .61 but was a bit of a dog even with good fuel and a pipe. Put a .91 in it and it's a whole new animal, on 15% fuel and a muffler. Now they sell it as a .61/.91 helo. The Raptor .61 is the same, it's now sold as a .61/.91.

Bottom line is subbing motors is a normal part of the evolution of helos.