Hi,
If your down in the tropic zone - as Tom Waits says - you got to find out from the locals first of all what sorts of wood they use there for such things, probably not ash n hickory, and many tropical woods can be very hard and brittle and not suitable for axes and hammers and such. But I'm sure you'll be able to find the local equivalent.
That said, that wood must be very dry to begin with, then using chisel, knife, I find a small spokeshave useful, properly fit that handle. Many test fittings will be necessary, marking the high spots and pairing them slowly down. I wouldn't grind the high spots by filing, rasping or sanding or anything, just shave them down carefully. At the end you'll probably want to wedge the handle with a kerf perpendicular to the striking direction. Drilling a little hole at the end of the kerf can help to prevent it going further. No need to glue that wedge in there, with a proper fit it will stay put.
You write of a pretty broad variety of implements you are wanting to put some kind of grip or handle on. I'm referring to here to a handle that passes through the implement like an axe for example. A scythe usually has some kind of integral clamping mechanism and so the fit is not overly critical unlike the choice of wood which must be tough like ash wood - maybe you could make one from rosewood.

Greetings

Don Wagstaff