I have an older Unisaw also. When I got my kickback epiphany, I was fortunate to have a bruised groin for several weeks, but no more damage.
My first act was to buy an aftermarket Beisemeyer splitter for the saw. Not as good as a riving knife, but it would have stopped my situation. To my knowlege, you cannot buy an aftermarket riving knife.
Plus one to all the suggestions about push sticks. The pic by Ole, is my go to for most things. I just used my hand saw handle as a guide and traced out a pattern, making several handles out of good plywood (no grain direction to split) routed all the appropriate edges and put a tall enough bottom on it so when it gets cut up I can slice it off and start over about three times.
The one push stick I rarely use is the common long and narrow one that you hold on one end. It is just too easy to put too much pressure on the single point of contact and make small pieces lift up or shift. When doing narrow strips, and just pushing with this kind of stick, the wind from the blade will lift the strip and start the reaction you are familiar with.
AKA Village Idiot.