There's a performance mod shop not far from me that is known for breaking horsepower records with their sleds. I had found them from a news article about their 650HP Yamaha Apex. Come to discover in talking to "Hurricane Dave" that was years ago, and their more recent efforts are in the 1000s. Of course, these are Yamaha engines in name only at this point. Virtually every part is custom made, specially mixed racing fuels are used, and *insane* boost pressures. Like 60+ PSI (Edge Performance, by contrast, uses only a ridiculous 40+). I definitely wouldn't trust a monster like that trans-pacific, it was obviously built to go fast, not to last long.
Interestingly though, he says the one part they've never had fail, even at those insane power levels, is the crankshaft. Tip of the hat to Yamaha for building a crankshaft that can take six times the force of its design spec!
Anyway, all this to say I think you could pull 180-200HP out of one of those engines naturally aspirated easily, and since they were designed for long treks at 75%+ power I don't think you'd have any issue. I personally would love a turbo just for altitude compensation, but the added complexity probably isn't worth it.
We make 313 hp on 91 Oct with this motor at 15.2 Psi of boost
Yamaha 160 lb engine
in Auto Engine Conversions
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We make 313 hp on 91 Oct with this motor at 15.2 Psi of boost