Update 12-02-2023. How to do it

 Update 12-02-2023. A suggested approach for Conversion to T-50-6 cores. [The blog post title does not involve Mary Jo @ 300 pounds and the back seat of a 57 VW Beetle.]] The Receiver from the Link A suggestion from a blog reader was to calculate the inductance of the coil and then make the calculation for the Toroid. The Coil forms in the article are Amphenol 24-5H. They are 3/4 inch in diameter and 2 inches long.   Another blog reader shared a 3 Transistor Radio Project that would work with the transmitter and can be found HERE The reader built this unit but substituted Hardened Cardboard tubes used for Model Rockets. The Cardboard tubes are sold by the Estes Company and are still in the current product line. He suggested the 0.98 inch tube. These can be found under the Accessories ~ Body Tubes!   In keeping with the plug in coil approach (likely you will not find the Amphenol parts) I think a 7 Pin miniature tube socket along with what is known as a tube saver (available on eBay)