It should be fairly simple to find a replacement adapter that fits your motor shaft. It sounds like a moot point now that you've ordered a new motor, but for future reference there are standard collet type adapters for every size motor available.
It sounds like you are just starting in the hobby, so if that's the case do you have someone who can help you with flying skills? A Decathalon in a nice flying plane, but it's not the most beginner friendly. Someone to at least do the initial trimming and talk you through your first few flights would give you a much better chance at success.