I generally use selicone sealent on the backplate and use blue loctite on the screws.

if the leak is coming from between the carb and the engine, i just jb weld it.
You can check the O ring to see if it's cracked or dried.

It might be coming from the barrel depending how old the engine is and how it was handle.
R/C car engines has a plastic boot on them becuase they really dirty.
A plane engine get dirty too if it get barried and not flushed.

makesure all of the screws are nice and tight. i purchased a use OS engine that was
runing eratic, all I did was tighten down the head screws that was a little bit loose
and engine ran great. I cleaned it a little bit then the seller wanted it back.lol