Usually the guys shorten the nose of the plane some to compensate for the weight up front with the engine.
Usually for a .60 size plane, they are using .60 on up two stroke engines and .91 on up four stroke engines, with the .91 to 1.25 size four stroke engines being popular. With gas engines many people were going with something like the DLE 20cc gas engine. So using a 21cc converted trimmer engine shouldn't be a problem. There are several people at our flying field flying 20cc gas engine powered stick style planes already.