I've been looking for the
Power Transfer Tyre P/N 560069 since earlier this year for my Masport Shiremaster, and I came across an eBay seller in Australia selling an aftermarket version for under $100. I bought one to give it a go, and it's been working pretty well so far. Thought I'd share the link below in case it helps anyone else.
Power Transfer Tyre P/N 560069
