Hello Dan
This is a common fault in my experience – both of my Scotts have suffered. I was looking at the drain plug on my 2 speeder this weekend as part of a rebuild. You will be able to find a replacement drain tap at autojumbles as they are not rare. In the meantime I suggest that you simply use a cut-down bolt. This does have the advantage of avoiding the possibility of some curious bystander operating the tap ( and draining the system) without your knowledge!
The thread on the 2 Speeder’s water jacket appears to be 3/8 inch BSF – at least that is the tap I used to clean it up. The drain tap I had needed the thead adjusting slightly by application of the appropriate die.
I hope that this helps – Good luck!
