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I was asked this question this morning. The thought is the nitro gearcase is smaller in size and a gear ratio better suited to run on a small tunnel or T boat. I remember several years ago Top Gun and Dave Muir were going to run an ETEC on a 13 foot Allison. It never was completed due to transom concerns. A fellow on Lake Muskoka had an ETEC on his Voodoo and it ran very well.
I believe the nitro case will bolt on. You likely need to rework the drive shaft.

Another question is can you make some of these newer motors that are only available in long shaft into a short shaft?
You need to look at the model you want to convert to see if it’s possible. Ideally you want a short shaft, with a smaller better gear ratio gearcase and if 4 stroke ...a motor you can reprogram/ flash to improve performance.

Yamaha did manufacturer short shaft 40 hp 4 strokes.
Here is the part number...67C-45111-00-4D ......NOTE...on a few models the part says short shaft, but on others it doesn’t....I wouldn’t just buy the part ...I would look for a complete used midsection so you know it is definitely a short shaft!
https://www.megazip.net/zapchasti-dlya- ... g-13528766

You do see them come up for sale. Many mercury 4 stroke parts are interchangeable with Yamaha. Those do it yourselfers with creative minds, patience and skill can build some very interesting combinations.

For the popular 56 OMC 2 stroke...those wanting a 15 inch, small good ratio gearcase and a factory power trim system are seeing a shortage in original OMC parts
An option that has great potential is the Yamaha 63D 40-50 hp short shaft. A few blown ones show up on marketplace every year. They are not that costly. That model has the nitro style gearcase with 1:85 gears , many came with factory power trim and the other Yamaha trim systems fit .unlike the 6H4 and 6H5 40-50 hp early model the 63D has a larger opening at the powerhead base. You should be able to adapt the 56’r to mount.

Too bad we can’t purchase new ones.....this is a tiller model but it shows what the motor looks like...you can easily tell it by the gearcase and the lower midsection mounts...good looking motor!
https://www.youtube.com/watch?v=baRY0B0VYyA

A nitro case will bolt onto a 60hp etec except for the driveshaft. The etec uses the same Multi spline driveshaft as a later 56ci motor. One could be made up from a 2cyl OMC driveshaft that would fit in with the Nitro gears..

I like this suggestion....the 1:86 gearing would be a performance improvement for sure and maybe a set of 1:71 gears could be even better.

So just to be clear if you wanted to up grade your Etec or your long shaft 70 hp Johnson or Evinrude 2 stroke these are the parts you need.

A long shaft 40, 50 or 60 hp Twin cylinder nitro style gearcase. They were on early 1982 to 1987 models. This will give you most of the parts you need and they are not that costly to purchase.
Example....https://www.facebook.com/marketplace/it ... 8e431d11d4
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You are going to need a 60 or 70 hp 2 stroke OMC 3 cylinder fine spline drive shaft. You only need the portion above the water pump. Marinas have them in the junk pile.
The twin drive shaft is a smaller diameter than the 3 cylinder, but they are the same diameter at the water pump. That is where you do the joint. Welded with a small pin insert would be good.

You can purchase new nitro case gear sets ..1:86 ratio. They are direct replacements for the gears in the gearcase.
https://gearprosinc.com/shop/ols/products/sst60-3

GP-SST60-178 (14:25 - 1.78:1 ratio)
GP-SST60-186 (14:26 - 1.86:1 ratio) equivalent to the O.E. ratio
GP-SST60-193 (14:27 - 1.93:1 ratio)

Mack Jones has solid mounts for the later model OMC triple if you want to add them.
https://www.facebook.com/jonesmarinedesign

Great update if you have an old style Voodoo with the 24 inch transom. The boat will be 5 miles faster and handle much better. With engine mods the boat will run over 70 mph.