Ethanol strips lubrication from the top rings and valve guides/stems. It is a solvent.
In the 750, with its steep valve angles, it wears out the valve guides quickly, in about 8000 miles in my experience, unless one is vigilant about adding oil to every tank of fuel. I managed to get 25000 miles from my 750 after ethanol 10% appeared here, by adding 1-2 ounces of oil to every tank of gas. Still, it wore the intake guides a lot. Installing bronze guides, as are found in cars engines made for the crap, works well. The 500/550 and 350F/400F have much shallower valve angles and don't suffer as badly from it.
BUT...it turns the fuel lines to hard plastic in about 12 months' time. Even the so-called 'ethanol-rated' lines (with the red stripe on them to symbolize "ethanol tolerant") lose all of their flexibility in about 18 months. There is some polyethylene fuel line out there, but in 1/4" size (a bit too big) that can help, if you can find some hose clips to seal up their weeping at joints.
If you rebuild, use only chrome top rings, too. This will help it last longer. Our friends at CruzinImage have lately (last 8 years or so) started doing this, too.