This is the reply I got from Stihl after asking if I could just buy engines;

Hello Mr. Cluff,

We are in receipt of and thank you for your message. Our engines are
manufactured strictly for use in Stihl manufactured products. They are not
sold separate of a completed unit. Thank you for your interest.

Sincerely,
Paulette V. DuPree
Co-ordinator, Customer Services
Stihl Incorporated



Sounds like it's time to go to the dealer and tell him my weed trimmer is broken and could he please order me a new engine for it.