well just my addition, i see no problem with fitting a second hand engine, if you do a few basic checks you have nothing to worry about, and it would be the easiest and cheapest option, much easier than stripping pistons out.
I have a perfectly fine G40 engine, thats been rebuilt, ive aquired numerous G40 engines that i have sold on, that have been fine, your assuming everyone selling an engine is selling it because its fucked.
You could easily find a complete g40 bottom end for about £150, swapping that is not a massive job either.
The dreaded piston meltage has plagued G40 owners for a long time, and no one has ever said exactly what causes it, i think because there is no direct cause, could be various things.