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The Yama/rude is Pro50 Yamaha trim on a 15 inch 49 or 56 OMC midsection. Lately T boat guys are wanted to have a better ratio gearcase on their OMC . The ideal gearcase is the small 1.64 ratio mercury gearcase. With modifications the newer one will fit, but it doesn’t look nice.

I think you’re going at it the wrong way. Why not mount the 56 OMC powerhead on a 15 inch Mercury midsection and run the 1.64 gearcase?
Obviously a 650XS midsection would be the best since it has top and bottom solid mounts. The stock 15 inch will work as well.

Why is it an easier build?
The top mounts on the mercury are in the midsection not as part of the motor base plate. The base plate mounts and bolts on to the top of the midsection.
I think a Kevi billet 56 base plate and tuner will work. You bolt the powerhead to the plate and drill additional holes in the plate for studs to bolt the plate to the midsection.
You will need to make a two piece drive shaft with a coupler.

Now the 52 and 59 cubic inch mercury 15 inch midsection and gearcases have a larger opening in the midsection and they could be easier builds. They come with factory trim, but are rare and expensive.

So how much of a performance improvement could there be over the nitro gearcase?
My guess is 5 mph with the right prop. Depending on the hp of the powerhead you will need a 22 or 23 pitch.

The later model 52/59 mercury cowlings should work and you end up with a real sleeper and it likely doesn’t cost as much to build as a 15 inch OMC nitro case.

Another option because they have the larger standard size clamp bracket and factory trim possibly is the later model 4 cylinder 40 hp mercury.

This is what one looks like.....it doesn’t have the 5 inch extension piece.
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Remember you could use an older 35 to 50 hp 15 inch midsection and buy a later model mercury powertrim clamp assembly and have the same combination as this later model.