I think that a factory remanned, Cummins 210 would be your best choice. Absolutely a rock solid engine with a simple turbo charger, no after cooler. Particularly if you buy from Seaboard Marine and let them upfit it with better auxiliaries. The foot print should be close to the Lehman since they are both about 6 liter inline sixes.
Might take a larger prop shaft depending on what you have and maybe a repitched prop to take advantage of the higher hp. But since the Cummins has a peak rpm that is the same as the Lehman you could just leave the prop shaft and prop the same. The Cummins will then pull the same hp and torque with the old prop as the Lehman since it will be under propped. But that is ok.
Not sure which Yanmar 170 the PP is referring to, either the discontinued 4LH-DTE or the new common rail 4HLV. Both are 4 cylinder engines. The former engine is also rock solid but with less displacement than the Cummins and you will have to look for a RTO and maybe rebuild yourself. The latter is a higher rpm (but not crazy high) new engine but with much less displacement which means it will rev higher and work harder than the Cummins.
David