The Historic line of race tires in 15inch looks to open up some alternatives for 15 inch Grp4 rears at least. I don't what would be applicable for the fronts. The All weather in 11/25-15 looks like a great choice. The caveat is the durometer is super soft but so are the TB15/5s. The Pirelli Corsa Classic W7 are 24.4 inches tall and these would be 25 inch according to the specs.

https://novamotorsport.com/products/...t-all-weather/