I haven't replaced the handle on any, so I don't know what you'll have to do to get the original handle off. However, I have bought the blades seperately and put handles on them. Its not very difficult the hardest part is just shaping the handle they way you want. I did one just the way you describe, get a block of wood drill a hole, widen it into a slot and then just epoxy the blade into it and shape the wood.

The others I put stacked leather handles on. First I tap the end of the tang to accept a decorative nut. Then I start with a sloted piece of brass as a simple hilt. I cut out rectangles of leather and measure them so that they each have a hole that matches where they'll sit on the tang. And I end it with another piece of brass plate. All the pieces are roughly shaped into ovals before I put them together. The first piece of brass is epoxied to the blade. The leather pieces are stacked on and glued together with wood glue. Finally the bottom piece of brass is epoxied on as well as the decorative nut (which I buy at the hardware store as switch for a lamp). Once its all together and set, I do the fine work. Shaping with a file or rasp and then sanding until its smooth. Then I coat it with shoe polish to finish it.

Hopefully that's clear in text, I can visualize the process perfectly, but setting it down is a little difficult.