(A picture of the broken piece would help.)
Option 1. (By far the best answer) Buy a new hacksaw. You won't be able to fix this without removing the threaded piece that still remains in the hole, then fabricating a new piece to hold the blade. This will involve adding threads to the new piece. If you don't have the tools to do this work, then you will need to obtain them, costing you more than a new hacksaw frame.
Option 2. Remove the threaded part that remains inside the hole. Usually this involves drilling a small hole in it, then you must buy a tool for removing screws that have been broken off in a hole. (Its usually called an ez-out, or something like that. A hardware store will have them.) You will insert that tool in the hole you have drilled. It will jam in place with a coarse reverse thread with sharp edges, and allow you to back out the threaded piece. It may help to drip some wd-40 in there as you are trying to back out the threaded piece.
Drilling that hole may itself be problematic, since there is a reasonable chance that part of the hacksaw was hard steel - by design to withstand the loads it must take to hold the blade. Worse, you must drill a hole into a surface that is jagged.
You may also be able to get the threaded piece out by other means, but you must do all of this without damaging the female threads in there, otherwise you will NEVER be able to fix the new piece in there.
As I said, unless you have a few important tools, this is all something that may be more easily repaired by simply buying a new hacksaw frame. Money solves everything, although a hacksaw frame is not that expensive. And if you don't know how to get that screw fragment out, then you probably don't have those tools, nor will you use them often enough in the future to merit buying them. At the same time, it is good to be able to repair things like this, so if you have the interest and the will to do this sort of work enough in the future, then go for it. But don't be at all surprised if you find it takes you considerably more time and expense than it is really worth to simply buy that new hacksaw frame.
You can use a Dremel cutoff wheel to cut off a piece of the rod but you may have to remove the rod to do this. You can use Dremel grinding wheels to grind down the end of the rod. If the rod is not protruding out of the hole, you could use a wooden plug to seal off the hole. Sand the plug flush with the bench top.
Best Answer
In general to transfer torque through an axle to a larger round thing there are three main ways:
Light loads. (fractional horsepower) A flat space is ground on the axle. A set screw in the hub of the round thing (usually a pulley) keeps the round thing from spinning. I've run into this a lot on trailer furnace blowers.
Medium loads: (1 to small number of HP) A keyway is cut into the end of the drive shaft and in the hub of the round thing. Typically about 3/16 wide by half that deep. The two are lined up, and a chunk of key stock slid in. A set screw keeps the key stock from leaving. Table saws, stationary power equipment are good examples.
High loads: (Tens of HP) You have a hub with bolt holes. The round thing is bolted to the hub. (Car wheels are a good example)
As is mentioned in the comments: This is not a good choice for a driven wheel. It's designed to have 60 pounds on a line through the centre of the axel. If you drive it, there are twisting forces on the ribs. I don't think you'd have to drive it hard for the wheel to collapse on the first bump.
You may find it educational to look at self propelled lawnmowers and see how they are set up.