I've only had need for left-handed bits when I was dissassembling Black and Decker appliances (I ended up with a boatload of broken ones) and there there were small screws, not rusted at all.
With right handed bits, I would start small and keep using bigger ones until I drilled out allmost the whole shank, the head fell off the shank, and the shank was close to falling out the other end. My cheap set of screw extracters didn't have one small enough to go into even the biggest of these little holes, and I don't think a good set comes with one much smaller.
With left handed bits it took much less time. I only had to drill once (which worked out fine since I only had one or maybe two drills smaller than the screw I was working on, and often when I was only 1/3 of the way in, the screw would come out. FTM, there wasn't enough for a screw extractor to bite on at that point.
I have no idea how it would work on big ones for an engine, but he asked the question, so I did my best to answer.
I can't emphasize enough that I think it is important to warm the engine to operating temperature or at least somewhat, regardless of what method is used.
Maybe when it is hot, and hotter still from the spinning bit, that would break down the rust and it would come out. I don't really know the nature of rusted in bolts. Is the rust weaker than solid steel, or does it literally bind the bolt to the hole? It seems like the rust might break down when hot.