trev
this all sounds a bit dangerous for an old timer like yerself ! ...depending on the thickness of logs, an alternative could be an all purpose saw with appropriate replaceable blade...or even a small chopsaw. Both are considerably safer and easier to maintain.
Having said that, if its chainsaws u want, a mate of mine uses a small leccy one to cut branches up to 100mm or so with ease.He has a garden wood burner and chops up pallets and tree branches etc.Its quiet and quite efficient .
As a builder I get loads of offcut wood...I have a garden chimnea which I feed with approx 9" - 12" timber off cuts( anything from 2 x2 to 8x2 etc) , chopped up with a jigsaw( timber only 2" thick) , then split down the grain into chunks about 2" square which fits the fire nicely , burns quickly and easily.
Trouble is, they dont take prisoners these chainsaws !