The info that Darock puts out is right on the money. There is nothing wrong with putting a larger engine on a plane, as long as you take into account a couple of things. Strength of the airframe, and balance of the plane.

Balance. Putting a larger engine on the plane will change to balance of the plane. You'll either have to move around equipment in the plane to re balance the plane, or add weight to the tail.

Strength of the airframe. This is a biggie. Especially the firewall. A bigger engine is going to put stresses on the airframe that it wasn't built for. Many times the firewall at a minimum, and the rest of the fuselage at times, needs to be made stronger. This usually isn't an issue when you are building the plane from a kit because you can evaluate every joint and part in the plane to see if it can handle the extra stress. But on an ARF, or a pre-built plane, you usually can't see the structure of the plane. While it's usually fairly easy to strengthen the firewall by pinning it and epoxying tri-stock to the back side you can't tell if anything needs to be done to the rest of the fuselage because of the covering.

With these points taken into consideration there isn't any reason why you couldn't put a larger engine on the plane.

Hope this helps

Ken