Sorry but I am traveling and have only limited/sporadic internet service. Also I apologize but I am not presently doing custom engine work. I just convert and hop up an occasional engine to sell or for myself. I am going through some extensive changes in my life plus recovering from two major back operations.
If you can locate one, a 23.6cc Echo from the late '90's is the best one to start with. Also if you find an Echo 25cc cheap or damaged, grab it as you can fabricate a cylinder spacer that will allow you to create a really good engine using the better 23.6cc cylinder and block along with the 25cc crankshaft.
AV8TOR