Hey Paul. Suggestion. go to the marineengine.com website and enter your model and year(s) and check part numbers for the block. Might be worth a try. If that doesn't work wondering if the splines on the crank and drive shaft are even the same.
Depends on many factors - checking the powerhead to exhaust housing gasket is a good start. Other factors include the splines as mentioned above, the ignition style, intake manifold style, and how much of the bolt-on stuff you want to transfer will make a difference, too. Not all 115s are created equally, either, as those post 1985 are actually the pre-1985 140s, as that was the year they went from powerhead rated to propshaft rated horsepower.