You might find that a 50hp 4-cyl lower unit of similar vintage would work as well. There is a difference in mounting stud width (the 2 studs sticking up at each side of the water pump), so you wouldn't be able to use one that's much older than mid-70's. I can't recall when they made the change, but all you need to do is measure the center-to-center width of your studs and make sure any L/U you look at has the same spacing.
As I recall the 60hp should have a driveshaft with long splines where it engages the crankshaft, in order to use the newer-style rubber seal that slides onto the driveshaft splines before installing.
So, any L/U you look at should have similar long splines. You might find that an older 44-cubic-inch 70hp Triple (or 65hp) may have the correct lower unit for your application, as well. And Merc did make some small-inch 50hp 3-cyl models up to at least the late-80's, a L/U from one of those should be a bolt-on.
As long as the shaft lengths and styles match up, and gearcase stud widths compatible, it should bolt right up to your midsection.
Craigslist is a good source of used outboard parts. Or maybe you'll get lucky and find a bargain on eBay.
HTH & G'luck.............ed
