The dr350 suffers the same problems with wear-quick cylinder heads. But it's normally down to the bike being run on low oil at some point  I would add that they seem to suffer more than most road bikes I've seen with similar sohc heads when ran on low oil.
I have an old head I got with a spares bike that has been repaired like this. One of the journals in the head bored out a little, and then drilled+tapped to take a grub screw which helps locate the bushing and keep it in place. No idea how well it worked though - The bushing is scored but it looks like the person who did it reused the cam which presumably had some scoring to the bearing surfaces.
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