Echo's are great.

Look at the BME engines http://www.bmeengine.com/ I like the BME classic 50. It is pure Echo. http://www.bmeengine.com/

I had a friend with a 55cc Makita on a Big Bee and there was no problem in the power department. It had plenty of vertical

That engine was done by Ralph Cunningham Who he had a lot of trouble with to ever get the defective ignition replaced by Ralph. It took over a year and sending it back twice to ever get another ignition out of him.