For the money, and for the power, a Briellelli would be a good choice. So would a 3mm 53 that Wild Hare is selling. Whichever is the least expensive at 50cc plus would be a good selection. Both are relatively light. Both perform well. Neither are what would usually be considered expensive for a gas engine in this size. Your plane may not need to fly nearly as much "on the wing" as you think. I believe you will be pleasantly surprised. Both engines are less expensive that the other "names" in the 50cc class. Both are reliable with good power and mid ranges.