Differences in Ryobi string trimmers

Hello, all.
I am researching this online as well, but need a fairly quick answer. What is the main difference between the Ryobi 700r, 750r, and for that matter 770r string trimmers? Do they all connect to the same attachments? They all appear to be 31cc two cycle engines, so I am wondering what the differences are?
Can I put a carburetor from a 750 onto a 770?
Should I just say screw it and buy a new one?
I have a 750 with a yanked (no jokes) pull cord and the clutch seems to have fused itself to the shaft. We have a 770 with a bad carburetor, and there is a used 700r for sale. In case you wondered about my questions.
Thanks in advance,
Maury French
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