THis may sound stupid but you are hammering from below yes?
The largest diameter rod or pipe you can easily slide into the steerer is what want to use.
A penetration oil may work to loosen any corrision better than WD40
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk