I want to weld a sprocket to the rear brake drum. Are there sprockets available that can do the job? Or do I need to find a suitable sprocket and machine out the centre?

Which bike are you talking about?...Ian

email (option): ian@wright52.plus.com

BSA m21 plunger that a QD wheel. I want to weld ring sprocket to the brake hub. I have taken the old sprocket off the brake hub

BSA m21 plunger, with a QD wheel

The brake drum and sprocket is available new for the plunger models. So why do you want to weld a new sprocket on? That is something you due when you can't get a new one like the non quick detach rigid model of the M21.

I am on a budget of sorts:) $200AUD + for the online option, to a $50 make it myself option, I have the skills and the equipment.

email (option): brycea662@gmail.com

You are lucky that the bike has a 520 chain size (5/8" x 1/4") as it is very common on Japanese off road bikes. Your M21 should have a 42 tooth rear sprocket. If it does Amazon has a 42 tooth 520 rear sprocket in steel for Japanese bikes. You will need to machine out the center.

Or try Martin gear and sprocket or Misumi.