Do they fit... yes but anyone who does go this route they may want to file or sand the area marked in red just a super tiny bit to make fitting the C clip back on with ease. I got it on but its a hair tight so a quick file and its perfect.
I'm still a fan of the original mod using the 2spd gears...
I'm sure you could have them made.
If you want to heat treat them, you need to get them to the color of the rising sun(800-900F), then quench in oil. You can then put them in the oven between 350-400 for about an hour to finish the process. Oh leave the gears in the oil until there about 120deg...
You can heat treat them and Quinch in motor oil. It will make them stronger. The savage gears are sinterened which is powder compressed at with high heat.