Ideally the primary DC resistance of your transformer should be around 600 ohms. This is fairly standard for battery type receivers like yours. You may either obtain an output transformer from a similar radio such as a Zenith or RCA portable that is in very poor condition, warranting sacrifice of parts, or you may purchase a new transformer. www.tubesandmore.com sells many transformers. Unless you have appropriate equipment for measuring your speaker's AC impedance, you will want to purchase an output transformer that has approximately a 600 ohm DC primary (you can go slightly higher or lower), and has a universal secondary with several taps. You will pick the secondary combination that yields the best output and tonal quality from your speaker. If a 600 ohm or similar primary cannot be obtained by itself, purchase an output transformer with a universal primary (primary has taps allowing you to select appropriate impedance). If you cannot obtain a transformer that has the DC resistances marked on it (allowing you to obtain 600 ohms), you will have to pick a resistance with your multi-meter. Usually the transformer will have the AC impedances, which aren't much of a help to you.
Generally, the lower the DC impedance (and concequently AC impedance), the more power you will get to a degree. The higher the AC impedance, the more bass response you will get to a degree. That said, when experimenting with output transformers, you will want to select a primary winding that has the lowest DC resistance without going below what is rated for the tube in use (and without increasing the plate voltage above that for which the tube is rated). At the same time, you will want the highest AC impedance available for a given DC impedance, to yield the best bass response.
Thomas