I noticed plastic planes like this don't handle the cold well. Especially ding ups in the cold. Is it possible that the stress cracks formed were in a colder climate?

Just a suggestion if you did not do this already. The Saito 72 is an expensive engine and I would run a steel fishing leader from the engine to a bulkhead in the fuse for safety. I saw a guy loose his wing on a float plane (I know what you guys are thinking - it wasn't me!). When the rescue boat came back, we noticed that every thing firewall fore-ward went to the bottom of the lake. I was videotaping at the time. The owner of the plane gave a copy of the tape to divers. The engine was never found.