The best thing to do is to pull the powerhead on your longhaft motor. Then measure how much the shaft sticks up. Compare that to the measurement of how far the short-shaft L/U shaft would stick up. If they're the same, you shouldn't have any issues with shaft length.
As you probably already figured out, they haven't changed a heck of a lot on these motors since the 60's. If you look up the powerhead gasket, it's the same part number from about '66 up to the last 4-cyl "Classic" 40 or 45hp in the 90's.
One thing my buddy did run into was, the studs on the '81 50hp powerhead he was using are longer than older motors, and the mid-section he had was an earlier one. So, he had a lot of stud sticking down. He solved that problem very simply, just used some spacers with the powerhead nuts.
But the mid you have, as I recall, was from a '78-or-so motor, and should match up with what you have.
All those 4-cyl 44-cubic-inch L/U, from the mid-70's and beyond, use the same stud pattern. Earlier L/U's have a skinnier stud pattern on the upper 2 front studs, but again, as long as you find a L/U the same year or newer than your motor, you should be find.
That '90's L/U with the beat-up skeg is the cheapest one on eBay, and if you'll check out the bolt pattern, I bet you'll find that it'll bolt right up to your Shortie mid.
BTW my buddy put that 50hp Shortie on a 10' GW Invader that he restored. Quite a kick and the boat planes out almost instantly. He's still getting it sorted out and installed a nice set of pressure-ram trim tabs, which stopped the porpoising of the hull. He had too much pitch on his prop and ordered another one, if he wasn't working this weekend, he'd be dialing it all-in at the lake! 80+ degree weather in W. WA this week, man is it hot! But it's over 100 in E. WA, I'm not gonna complain too much!
That skeg could be weld-repaired, if you know a good TIG or MIG welder they could fix you right up. The L/U is the newer swept-fin design, which is more desirable and has better handling characteristics at higher speeds.
Shift shaft might be the tough one. BTW you can use an older shift shaft, it'll have a different connection to the shift arm, so you can't use the one from your engine. But if you order the older arm, it bolts up just fine. That's what my bud did, we found an NOS part on eBay and it fit perfectly.
There's one on there right now:
http://www.ebay.com/itm/321336252822
You can use the exhaust tuner plate out of your motor. And as I recall, you can cut the exhaust tube, in fact there's a joint where Merc added the extra length for a longshaft. Just make sure to measure how far down the exhaust tube sticks into the longshaft mid, and ensure you cut the tube so it's the same length sticking down in the short mid as well.
Same goes for the water tube, it can be cut to the shorter length, as you'll see the bends are compatible. Just do the same as with the exhaust tube and measure where the tube sits in the lonshaft mid; it'll be the identical dimension as it sits in the shortie mid.
Maybe you already found those parts and don't need to chop anything, even better.
Cheers.............ed
