After reading your post again it makes more sense! I’d definitely love to see how you test goes!
2 strokes have a large rpm range from not to 20,000 in some engines. They have an optimum rpm for power that varies but is usually somewhere around 7-8,000
This topic got me interested and now I’m looking at ways to do this for a tow board.
Engine: Pocket bike 50cc pull start. Very cheap on Alibaba/Amazon. Weight only 3.3 kg!!
Exhaust: Surfjet uses a simple ball and spring to provide back pressure and act as a seal to keep water out of exhaust (info here thedailywithfish.wordpress.com/2017/09/15/did-you-know-the-waterboxes-are-different-jetsurf/)
They are for sale new on jetsurf website.
I think best for a tow build would be to buy a wake surf board as a base and build a box on top of that for initial testing.
More ideas guys! Love this website.
Do you know what the ball is made from so it doesnt melt? Ceramic?
No idea, cheap though:
'https://jetsurf.com/collections/exhaust-system/products/p004515
Since the ball is 98% of the time in contact with water, it’s water cooled 
More seriously:
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I think rpm is less important, since there will be a smaller propeller, and high rpm will cause it to cavitate. In eFoil topics, usuall rpm is under 5000. In GX50, the max rpm is 7000, so I will use 1:1.5 90° bracket, which should give me more low rpm torque for easier lift off. Given the 4 stroke has good low rpm torque, it should be more slower-cruise friendly
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Gas foil will have exhaust in air, so there is no water cooling, and 2 strokes can get really hot in no time, up to 600°C with high rpm. But since BigJilm is using a waterpump to watercool the engine, the “warm” water can then be used to cool exhaust too and then dump it out hot, muffling the exhaust noise in the process
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I wondered if the Honda 4 stroke GV or GVC series engines could be used, they have a vertically down shaft and come in various different sizes, maybe the GCV170 could be suitable?
Cons
- They are heavy but still within an acceptable range
- 4 stroke so needs to be kept upright? Foils very rarely go upside down in use but the transport to the water will have to be reinvented?
Just brain storming 
Just putting something down to look at. The motor could be deeper into the board but I would imagine you would have to mount the larger board from the side like a SUP.
This shows a solid 10mm drive shaft through the mast with a suitable cutout where a bevel gearbox would sit and re strengthen the area. You could have a 3d printed hydrodynamic cover around the gearbox to smooth everything off.
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This is going to be very interesting!
The noise and the smell folks. This type of build is not worth pursuing, too much time spent to get something inferior in the end 
Speaking of high costs for e-drive, then comparing a relatively simple electric direct drive with low quality outboards isn’t really apples to apples either. and when were ICE engines superior in torque? Never…
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Definitely worth pursuing in my opinion!
Pros:
Cheap, I have all my materials for my build and have spent less than $500
Unlimited ride time, just refuel instead of waiting for batteries to charge!
Light weight, far lighter than an efoil.
Cons:
Definitely noisy, but hey my outboard on my boat doesnt bother me.
Less torque
[UnEcological mode = ON]
With a well designed system exhausting at mast length below the water level, the smell could be for the fishes …only.
[UnEcological mode = OFF]
I’m totally ashamed to have written this… 
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I spent 1300$ for my complete build, ready to fly.
What is included in the 500$, everything?
Who doesn’t like the smell of a knackered mercury 4 stroke 
?
I prefer the smell (and sound) of a 2 stroke more 
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I have no intention of building one because electric is obviously better but I like to throw ideas out there and think how things could be done.
So, any ideas on what kind of off the shelf bevel gearbox could be suitable?
From an angle grinder, or polisher, they should be ok for high rpm. Question is if they support the torque. Will be loud as well but this does not matter in this case…
The most sensible way to do it would probably be to use a 1:1 right angle gear low down in the water and then use a reduction gear in the board to gain torque. That way you can keep the pod in the water small. A stainless shaft with Acetal gears would keep experimental costs low and would have less issues if water got in.