Of the two engines you asked about the 40cc would be the better choice for the airframe mentioned. The 30 will fly it but it wouldn't be impressive, BTerry's advice is spot on as is T.O.M.'s. But, if you want or need to go with the less expensive engines, they do work very well and if purchased from a reputable distributor any needed support will be there whenever it's needed. If you go with the CRRC 40 I would recommend upgrading to the Rcxel ignition.