If it was me, I would get the original engine looked at. You would then get the worn parts replaced, ( At the most pistons, rings re bore etc) Even get it bored out if that was your thing.
And you would have piece of mind knowing that the engine was reconditioned from the crank cases up. Might even be worth getting the rest checked while it was apart.
OR
You could buy a used lump but no matter what the seller tells you, you would never truly know much about it.
I mean that it might be bolted in, fired up and a month down the line, be burning oil just like yours is now!!
I know what I would do.................
DC