Running a gas motor you do not need a BEC you can either run 2 2 cell Life batteries one for ignition and one to the receiver or one larger Life. Some like to keep igniton battery seperate as there is a small chance of interference from the ignition to the receiver.You should really run an Opto switch with a gas engine for safety reasons. You do not need a special servo for the trottle any standard servo works, you will adjust linkage and throw by the length of servo arm you use then set up the rest in your transmitter so you do not stall the servo...
Edited to add I would not of painted the cowling yet as modification are going to have to be made for the DLE to fit for cooling, and exhaust.