RE: P-51 Mustag
Of course... you'd have to appropriately beef up the airframe to handle the larger engine. If the structure is made for .051 to .10, then the .25 or the racing engine might pull it apart.
You say its a kit.... not worked on at all yet? How you go about it can make a huge difference in how it handles high speeds. (and how fast it can go due to drag)