Can’t picture it, but on other silencers I have cut out sections & run perforated tube before it gets to the spark arrestor & packed around that to quieten them down.
You should be able to buy the perforated tube cheaply, perhaps from a muffler manufacturer in the right sort of diameter to match up & weld or jam it in place.
Try to leave the arrestor in for obvious reasons & the reversal of flow will quieting it a few dB. An endcap with another reversal onto the end of the muffler may help as well.