Hans,
My tow plane is 19 lbs, and clunky at that. With the 35cc on a canister muffler I can fly aerobatics, and have a 70 degree climb-out. (20x8 at 7200 rpm static) If the muffler is chosen such as not to restrict the power further, the engine would do just fine.
With a very restrictive in cowl muffler, power will indeed be low, and not allow a 45 degree climb-out.