I did the same with a no name bolo with a two inch thick black walnut branch sawed in half to make slabs. After I had the two halves sanded flat on the inside plane I trimmed the outside thickness down from the radius edge of the branch then cut them to approximate length.
Take your machete which should be bare handled and cleaned of any rust, dirt or epoxy then mark out your rivet holes using the holes in the tang. Drill them as wide as your rivet material (brass rod, cutoff bolt, mystery rod) starting with one slab side first then tape the two slabs together to use the holes of the one side to guide your drilling the second one. Leave yourself plenty of overage on the scale slabs out over the tang, you can trim, rasp, or sand that down after everything is epoxied and rivets together.
Once your slabs are drilled out to rivet, check them again on the tang to make sure they look right. If you rivet with copper or brass you can file then sand the exposed rivet flush with the wood. With this method you use epoxy, many brands to choose from, pick a slow setting type since they have better grip. JB Weld is a strong one but looks butt ugly if you get sloppy and don't sand it off later, there's many other types of epoxy. Line up your scales, epoxy the slabs getting some into the rivet holes and tap the rivets in taking care to not rush, an advantage with a slow grabbing epoxy, then clamp it all up to set until the epoxy label says that it should be ready.
12 or 24 hours later take the clamps off and start in on final shaping and sanding. Fit and finish are only limited by your time and patience. Look at some other machete handles that seem good to fit your hand and try to get yours to look and feel like them or better.
This is probably as basic an explanation as you can get, there's many other ways to do it so look at other ideas. You can practice on any good idea with cheap pine to get a feel for better materials. You could use cutoffs of maple or any other hardwood you can find, watch for trees being cut down or brush piles that have been left in places you are allowed to use wood that is down already like you would for camping firewood (better to use seasoned dry wood over green).
You should end up with a tool handle you can use and not have it mess up your hand like it used to do. You can do it, whether it looks like a crude farm tool or finely made collector's piece is up to you. If you are able do some pictures for us...that's from the guy who never puts pictures into anything I post. LOL
. So Ive decided Im going to make new handle scales for it. This is something Ive never done, so I need some recommendations. What material would be best? I would like to use g10, but would micarta or wood have any better advantages? Also, whats the best way to attach them? I still have 3 of the rivets that attached the original scales. Any help is greatly appreciated 
. Thanks for all of the help everyone, i really appreciate it especially since its my first major modification.