Something does not make sense and I don't see any root cause analysis being done as to why your units are crapping out. Just saying they're cheap doesn't cut it.
A lot of people have been happy with these. As long as the engine can put out power and the electronics (capacitor?) works, I don't see why it should not continue to work. There is not much to them. Why would copper winding give out after time?
For a 2-stroke, after about 100 hours don't you have to clean out/decarbon the exhaust?
What is your evidence that it is making less and less power over time? I don't see where you mentioned that. Is the engine not running?
