I woulkd stay with the 50cc class of engines. The 60's will be more weight and the increased power will not offsewt the weight penalty in wing loading. DA, Brison, BME, Taurus are among the first I would consider, and with the exception of the Taurus, are relatively comparable in power output. The Taurus being a touch more in that department, IMO. The price of the engine and it's relative all up weight will be the determining factors.