I might add that many of those string trimmer heads will interchange on a shaft. if you can find a scrap yard that will sell stuff you might find source of parts for cheap.
I have long experience with string trimmers. It was like 15 or more years ago that I got completely and totally frustrated trying to get a string trimmer running from my shed, Ryobi, echo, toro, and a few others were in the shed from over the years, spent a bunch on parts to try and fix them and none would start and run consistently . completely frustrated I went to stihl dealer and spent a whole bunch on a new km130. that powerhead still starts on first or second pull of the rope every time I pull it out to use it. Not sure how many hundreds of hours its got on it but still has the same full power as when it was brand new. Well worth the $$ it cost and has paid for itself many times over.