I understand the reason you've given Ian, but why not just buy an off-brand full-crank trimmer? I've got a classic Echo and a modern generic full-crank 25.4cc thing sold under half a dozen brands, and I only use the generic, not the Echo, because it is so much nicer to use and gives so much less trouble. The generic cost me $1 on ebay, and took 90 minutes to fix, using no new parts at all. It didn't even need a tune, because it had only ever run for a few minutes before it went wrong.
I doubt you care, but what went wrong was the previous tenant left it out in the sun, and the plastic axle the pull-starter reel rotates on, grew a bit. That meant he couldn't pull the starter, so the guy gave it away to another guy who put it on ebay with a warning that it looked as if it had been run without oil in the fuel, because the engine was seized. Oddly enough, no-one but me wanted to pay even $1 for it. All it needed was dismantling the pull start and sandpapering the shaft with a finger-tip until the reel rotated easily. Hopefully I'll have the sense not to leave it out in the sun, but meanwhile it is the nicest trimmer I've ever used, by far.
