Once the seized shaft is out it may need replacing... whether you buy or fabricate from rod using a Die you need the correct Diameter and Thread Pitch.

ChgImposs Metric & Imperial & US Screw Gauge SAE Whitworth 55 Degree & Metric 60 Degree & & US 60 Degree Thread Pitch Gauge for Industrial Measurement
5.0

52 Pcs Stainless Steel Screw Thread Gauge Tool, 60°/55° Metric and Imperial Thread Pitch Cutting Gauge Screw Gauge Measuring Tool
5.0

40Pcs Thread Gauge, Thread Gauge Metric and Imperial with 55 Degree Imperial and 60 Degree Metric, for Threads Pitch Measurement
5.0