OK, where do I start? The hull might take the extra power for a while but it's pretty thin for any serious .21 motor. You'd have to modify the motor mount. The driveline should be upgraded also as the little 1/8" shaft ain't gonna get it. You won't be able to put a large prop under the hull that you're going to need. Converting to a surface drive set-up would be he next step. Yeah, you'll have to toss the tuned pipe out with the motor. don't forget to install a larger fuel tank, you'll want 8-10 oz. in there.
The radio box should be fine if that helps at all.
Alternative. Get a killer .12-.18 motor and it'll bolt right in. Just remember, fully threaded crank and rotary carb then get plenty of air to the heat-sink.