Output transformers are impedance matching devices and winding resistance has little to do with the specs that are important to know.
If in fact your transformer is bad, then start by listing the important details of the amp. What design type output is the amp? What output tube does it use? What is the secondary speaker load?
If keeping the amp "as true as possible" is the important factor here, then search the web for an original part, or order a Mercury clone. If getting the amp up and running is the main factor, then search for a similar transformer from another supplier.