The arm has a bit of a slot on it to hug the machine and line up when you screw it on, so I don't thing a shim would work. I think it must be a little bend at the top, so there is about a 3mm offset when looking online with blade.
Please bend back in line or get a new one. It makes me cringe when I see YouTube woodworkers not using a riving knife. Yes, it can be done safely but why invite the possibility of a piece being launched at your face.
When I was an apprentice, we had to use a panel saw with no riving knife. Removed and lost by the shop foreman, total clown. Fortunately, never had a kickback.
However, one day I cut a piece of 3/4” blockboard. As I went to move the piece off the outfeed table, I accidentally nudged the off cut into the rising teeth.
It was a big saw and a big offcut, which hit a set of drawers about 12 ft behind the saw. The drawers were about 5ft tall and had a huge dent at the top. I dread to think of the damage if I’d been stood in front of that :|
My mistake but a riving knife would have stopped that incident happening.
I've assumed table saw amputations are non fatal, so the upper limit ('cos some are legs and toes) is the total number - 573 in 2018/19 - in a population of 67M. Rate per 10M population = 86
Now, perhaps US folk have wider ownership of table saws, or use them more often, or are more diligent about reporting stats, but the stats show at least a 65% higher rate of table saw amputation in the US than total amputations in the UK.
Two final points:
1. Stats are, by their nature, an accumulation of individual anecdote. It's unlikely to be you, but it shall be someone. And it might be you...
2. Given that it might be you, and the effort required to materially mitigate the risk is quite small... fix the knife.