So once again I will preface all this with the comment that I am not a pro with this stuff but have had some significant exposure. But please DO NOT take my advice as as any sort of last word. I am sure there are a lot of details I am overlooking here. You have some homework to do.
With that said, here is my take. You can make a hydraulic motor run at any speed you want. Think of a machine like a skidsteer Bobcat that utilizes a hydraulic drive. . The engine is running constant speed at maybe 2000RPM driving the pump, but the hydraulic motors can be driving the wheels at just a few RPM if you are creeping it along.
To get the required pressure and flow, the pump needs to turn at some specified speed that is determined by the design of the particular pump.
So long as the pump is supplying enough flow & pressure, you can vary the flow to control the speed of the motor. That is essentially what the hydrostatic drive on a tractor does. The pump runs continuously at whatever speed the engine is running, and you vary the flow to the hydraulic motor to control the speed of the tractor. More flow = more speed. Because the pressure is always pretty much the same, the torque you supply will remain the same as well. So in theory, you can have full torque at zero (or near zero) RPM. Of course theory and the real world are not quite the same thing, but it's pretty close to that. This is why things like skid steers and tractors use hydraulic drives. It gives you the ability to control lots of power quite precisely. The hydraulics serve as the gear box, essentially.
There are subtleties involved of course. You can't just valve off the flow from the pump, it is either diverted or you need a variable displacement pump, etc.
I think that what you want to do should be within your capabilities with a bit of research. I don't know what would be better, the small engine or a hydraulic drive. Like any engineering decision, it is a series of tradeoffs. No doubt either approach can be made to work well, they each have different strengths and weaknesses. The application and your preferences will dictate which strengths and weaknesses are most important.
You may want to take a look at the surplus center website (I think someone may have mentioned this already) at their technical info page here:
https://www.surpluscenter.com/Tech-Help/Tech-Help-Home/
They link some helpful videos and have calculators about hydraulics to help with the design work. Lots of other info on the web as well. If you prefer to read a book, this is a decent basics book that will get you in the ballpark:
https://www.amazon.com/Hydraulics-ro...p;qid=&sr=
I am sure there are lots of similarly good books out there. Hope that helps a little.
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