Peter: If you're keen on an M23 and your competition rules allow non-Scalex cars, you could look at an Airfix or a Polistil/Policar.
The old Scalextric M23 was undersized, with a short wheelbase. I think the Airfix came with a Johnson 111 and was to a better scale.
The Poli M23 that I have has an S-can motor, but two others in the same series have the Johnson 111, so maybe some M23s had it too. Even if you found one with an S-can, it would be possible to modify it to take a 111.
The Poli model is a little over-scale, but still within Scalextric variations, and you'd have a reasonable wheelbase and track. It has a lot of ugly "chrome" plating at the back, but it could be made to look quite good.
This range of Policars was also marketed in North America as Strombecker, and over the years some found their way to other countries, so if you see a "Strombecker McLaren M23" it would be one of these.
What are you actually trying to do? I have two C127s and would happily part with one (or both), but C127s must be common in your part of the world, and presumably you're aiming for something better.