Here is a link to my post for an Echo CST-610 twin I converted. Made between 1981 and 1984 if I remember correctly. They are pretty hard to find. The ones I have were basket cases meaning they were not running. But hey all we need is the short block anyway. I do have one running saw but it will not be converted. Only "issues" with them is the carb ends up on top of the engine. Makes for challenges with cowling unless you design or adapt a muffler to work. Other cosmetic issue is one plug points up and one down. The carb is on an angle in the saw but you can make a plate to mount it upright.

http://www.rcuniverse.com/forum/m_44...tm.htm#4413974