RE: Lambert kolibri
I have flown the Kolbri in a plane weighing 5.5 pounds (I can't remember if it was with or without fuel). It was the Bobcat 25 from Nitro Planes. That is about the limit I would go. It pushed it fine. Vertical was obviously not unlimited, but it was faster than scale speeds. That L-39 sounds pretty equivalent. They are a blast!
Chad