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.

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

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

Original Thread Checker (Inch & Metric) - Size Gauge Measuring Tool for Imperial and Metric Nuts ,26 Male/Female Gauges - 14 Inch & 12 Metric
5.0