I've just purchased a cub frame for the Stafford show, does anyone know what size the steering head outer race should be

It’s the later type with the wide swing arm

I think its been modified on the frame I purchased?

Sorry, I don't know the exact size but I had to grind off about 1mm from the OD of a tapered bearing to fit them to my Cub steering head. It's possible your bike has had the frame machined out instead of the bearing od reduced.

I'd measure it and see if you can get a tapered head bearing (from ebay?) to fit.

Hope this helps,

Bruce.