First, go with a 50cc plus, not a 45cc. More aircraft options later.

Second, there are a lot of great engines out there in this size range. It will come down to fitting your budget. Fox, First Place, Brison, BME, 3W, Taurus are a quick few I can think of, and not necessarily in that order.

Third, that "burbal" you hear about is directly controlled by the needle settings input by the user, and the stage of break in the engine is at.