Well, the 15hp and the 10hp shared many of the same midsection parts in 1956, and I’m pretty sure the base gaskets are the same. That being said, one of your pictures seems to show that one of the 1/4 screws that is supposed to secure the powerhead to the midsection is missing, so you may want to investigate that. Perhaps the screw is just missing, or maybe the threads in the powerhead are stripped, or the original screw may be snapped off up there. This issue can cause an exhaust/cooling leak, so it is worth looking into.
Like I say, the 10hp and the 15-18hp shared the same midsection/gearcase in 56-57, so I don’t see any harm in using this engine “as is”, except for the missing powerhead screw. In 1958, the 10hp adapted to the smaller 7.5hp midsection and gearcase. So, I would treat this engine like any other used engine. Do the basic tests. Check compression, drain and inspect the gear lube. You should probably remove the gearcase and change the water pump impeller as well. Check out the ignition and fuel systems. This engine originally required the two line fuel tank, but someone may have converted it to the single line system with a fuel pump. So, if you have the original two line fuel tank, you will need to inspect inside. It isn’t much good if it is rusty/fouled inside. The 10hps had another issue to look at, and that is wrist pin slop, they had no needle bearings on the wrist pins. So, you might want to check for wrist pin slop as well.
Needless to say, I wouldn’t put too much money/time into this engine, especially if it has mechanical issues. On the other hand, you never know, perhaps it will run just fine with some normal servicing.
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