Regarding Lower Units, any L/U from a '65-and-newer Inline Six will fit your '72 140HP. The one difference you're gonna find in later models is the lack of an O-ring groove in the driveshaft splined area where it engages & seals the crankshaft splines. If you need this groove for your motor and you find a later L/U that doesn't have one, you can grind the O-ring groove yourself.
Starting around '76 the driveshafts were stainless or stainless/clad in the wear areas so these are definitely more desireable than the older units with steel driveshafts, which were susceptible to corrosion in the oil seal area.
Note that 66 c.i. Inline Fours of similar vintage have the same style L/U. Later model 65HP's have 2.3:1 gears, as do all 75, 80, and 85 HP Fours, and '70-71 115's and 135's have 1.78:1 gears. These may not be an appropriate gear ratio for your 140, which came with a 2:1 gearbox. Not that they wouldn't work, but will definitely impact prop selection.
Inlines other than those mentioned have the 2:1 gears and it's easy to check, just count driveshaft revolutions for every one turn of the propshaft. You can do this in either Fwd or Reverse to check.
Other differences in design of the bearings and gears over the years make no difference in fitment, so what I'd suggest is find the best deal on the newest one you can get and let 'er rip! Lots of deals on Craigslist and you may luck out and find a whole parts motor on there for cheaper than you could buy a used L/U on eBay, for example.
Lower units from the newer Big Threes and Big Fours are not compatible with the older Inline bolt pattern. So your friend's 75HP with the blown powerhead won't work.
From what I understand, the PT&T section may be possible to retrofit, but you'd have to swap out the entire steering yoke/transom mount assemblies, which would involve pulling the powerheads on both motors. The newer-style integrated PT&T units are real nice but you'd have to measure the mounting bolt spans (width of motor mount bolts top & bottom, and bolt spacing side-to-side) to see if they're the same. I've heard of this being done but haven't personally done it yet. I'd like to find one since I have a nice 1400 powerhead sitting on the bench to build. The integrated unit makes for a much cleaner installation, since you don't have any external hoses or pump to mess with.
Here's an older-style L/U which looks like it has the stainless-cladded driveshaft at the pump area:
http://www.ebay.com/itm/180777616163
Here's an example of a newer L/U that would retrofit to your 140:
http://www.ebay.com/itm/130618701777
Note the swept-back design, which was an upgrade from previous gear housings. '79-up were like this. I used a similar L/U on my beloved 1350 and it worked great (wish I had never sold it!).
Your 140 is more-or-less a 1350 with some tuning improvements on the exhaust side, a very fast motor on the right hull.
HTH & Have Fun!.............ed
