Gun, You have several choices. The temper now where it is is about just right for a knife like that. So you can work it cold. You can cut the shape, drill the tang and sharpen it, just don't get it hot.
The steel will be just slighty harder to work as is, than the other option.
Heat the steel in a fire, like maybe in a wood stove till it is dull red hot in low light, then bury the steel in ashes to cool over a long time about 8 to 12 hours.
This will have you at dead soft and annealed, soft as it gets. Then cut the shape, drill the tang hols and profile to sharp.
Next you need a container with a oil quench, and long enough the blade can be dipped horizontally, like maybe a old steel window box for plants if you can find. The blade must be heated to a bright orange in good light, and or untill a magnet no longer attracts. Then quenched rapidly moving the blade up and down to shake off boiling bubbles. This is more or less a 1.5 second dip, not all the way to cold. Watch out that you don't have a flash fire. I reccomend used canoila oil, for it's better food like smell and higher flash point.
The blade will reach a lower but still hot temp, ya gotta be movin real fast with a vise pre set to straighten the glass hard blade if it warped any. The steel being very thing, you could dip about 1/2 the blade or up to about 1/2 of the blade at the working edge.
All the cooked on oil will need to be power wire brushed, and this is a little dangerous since the power wire wheel can grab the blade, and fling it, and it will be already pre sharp.
Another heating to draw or temper comes next. I like to do a batch of around 6 blades or more for this. I use a normal kitchen oven, preferably cooking a roast or a turkey. I use a store bought independant thermomitor, and cook the blades at the same temp the food requires for that amount of time and when the food is done you have soaked that much heat into the temper all the way thru the steel, and made it more flexable to a point. For some blaqdes this is enough, but I doubt it wll be as flexible and spring like as a fillet knife should be.
There are more options. The easiest is to set the oven to 600 if it will, and allow the time to be around 30 minutes, for one more heat range hotter,at which when the blade has soaked in 600 degrees should appear peacock blue at the correct temper for a fillet knife.
Another method would be to heat something like a brick made of steel to that hot and lay the back spine of the blade on that and watch the 'parade of color' run to the sharp edge.
yet another would be to use a propanr plumber torch and 'paint' the flame on the spine of the blade to see the color become that same peacock blue.
Painting with flame is hard to do, and you must be ready to quench of the color passes that blue. If the color goes to a dull gray you have normalized the steel and made it too soft again.
If the blue runs and look as if it will stop at a shade of purple before the blue comes to the cutting edge, that is fine. That means that edge is a tad harder, which will hold a sharper edge longer, and the blue color will still be flexible.
Flexible can be the abilty to absorb shock as this is the same temps and mannor a cold chisel is made by and you are not going to bend any cold chisel, but you can wack it and it won't shatter.
Lance points I make are this same way to and over the length of a horse shoe rasp they will bend and spring back straight.
What makes this hard on a fillet knife is the steel is so very thin to start with.
The blue can be sanded off with finner grit papers, or be left as is. The parade of color can best be seen wasting a common stainless steel butter knife. You can't really harden ss, but you can see the colors.
With a junk like knife grab the handle, set a plumbers torch to the far tip, and heat it till the tip begins to turn a light or pale straw. Since this type of knife is thin too, it won't take long to see the colors start. Hold the flame loosely on the tip never doing more than wave the flame left and right from the tip, and once the pale staw begins to darken remove the heat.
You will see the colors parade up towards the grip. There is a good bet that the tip where the flame was will end up looking dull gray, which is another way to get dead soft in carbon steels, the straw will have traveled towards the grip, and many other colors showing between, each with it's respective heat range, all of which are showing softer to harder, in degrees.
Frankly if I had the start you have, I would do it all every bit dead cold. With a whizz wheel I would cut a little and dip in cold water, cut a little more scoring the saw blade not cut thru and dip in cold water. Once the score was done I would snap off the weakend joint to have a rough blade, never once seeing red hot, never once getting metal close to the cut hot on my fingers.
This is to retain fully the temper the saw blade already has.
The heat for drilling rivet holes means nothing, since it is heat at the tang.
The heating methods offer a great chance at cracking the blade when the steel is this thin.
If this happens it may be possible to salvage the point, for another project, and this does happen.
I have a knife today 1.5 inches long over all, which was salavaged from a belt knife I made that cracked in 3 places in the first quench. That little cutter is some wicked nasty hard, but it was pre sharp, so the sharpening I did once, allows that to slice 9/16th" elk neck like a knive in hot butter.
I use this for making mocs,cutting the patterns and to trim threads sewwing. It lives in the hem of my Balmora hat
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