So I went back and looked at my notes. First, you can press the shaft off. Gears are pressed onto a splined/swaged shaft. I have replace gears on diesel trucks and other engines. The key in reassembly is location of the new gear and using spacers to ensure it gets positioned correctly during installation.
The new part assembly MTH came up with is now TP-OE00147. This is a complete shaft and gear assembly that will not require pressing. I have one in stock if you needed it.
To get the old shaft off, you could press out on the side opposite of the idler. Use a spacer to keep the damaged worm gear from moving as the shaft is driven out. G