Jim does that Echo have attachments? What I mean is, the flex shafts aren't all that different between brands. Ryobi used to sell the Quick Attach coupler as a separate accessory. The only difference might be the coupler itself allowing you to use your Ryobi attachments.
I've replaced the shaft w/coupler for mine on different powerheads. Hate to see those attachments go idle when/if the Ryobi quits.
And yes, those pre-MTD units were/are the best. Whenever I find one at a garage sale it's usually pretty cheap. And the owner usually throws in the attachments he has. A primer bulb and new fuel lines usually get it going.
I have a 725, 775 and BC30. And too many attachments. I bought the 725 new in 95? Still going strong.
I think MTD messed up on a bunch of the 4 cycles, esp. the TroyBilt. They used the 2 cycle clutch with weights too light, which kicks in at a much higher rpm than it should for a 4 cycle. By then, the 4 cycle is producing way more torque. And snaps the shaft.
The 2 cycle woulda moaned a little and built the rpms back up.
Ted