If what the dealer is telling you true, their compression measurement of 75 psi is way too low for your quad which should have minimum around 150 psi (up to 220 psi if really healthy). It sounds like the crank got tweaked when it hydrolocked the cylinder. If that's the case, what their estimate is about the right cost in my area because the new crank costs about $375. You may want to look for an used motor from crash unit or something.