You have several issues at play...
A typical plate modulation transformer has a secondary impedance of around 5k to 7k depending on the PA tube(s). It is possible to use a plain vanilla power transformer or audio output transformer - except - you'll be running them backwards.
Typically a transformer is wound with the primary nearest the core so as to induce maximum flux in the laminations. In some cases the primary and secondary windings are 'stacked' equally around the core like two Pregnant Pi windings and would be preferable for this application. More often than not the secondary is wound on top of the primary. You should have a power transformer like that in your junque box with a 6 or 12 volt secondary and 120 VAC primary. Try the secondary across the amplifier with the primary in line with the B+ of the EICO PA section. Might work. Likewise a tube output transformer in the 7k range with a 3.2 to 8 Ohm secondary wired backwards might do something, but remember the pseudo-primary will be shielded from the core so I wouldn't expect much efficiency or a reliable impedance match when it's bass-ackwards. Also, with any modern lo-volt power transformer remember the Hi-Pot rating might be only 300 or 600 volts... Not the 1500 more common back in the day.
Ultimately you may end up keeping an eye open for an older PA amplifier with a nice beefy output transformer capable of 70 volt (600 Ohm) output. Research this and you'll find plate modulation is possible with the 70 volt winding in series with a second inductor. You'll find big iron in both tube and silicon PA amps and the worse shape the better as you're buying it for the transformer, so uglier is both cheaper and nicer. Sorta'. Iron like that runs into serious coin if you try to replicate the matching EICO modulator using new parts.
If you've been known to look at schematics for brain-fart fodder, here's a nice assortment of things PA:
http://www.epanorama.net/schematicsforfree/Audio/Products/PA/index.phpI'm partial to older Dukane amps, but a Bogen is easier to find and research................
BTW: Here's something to consider for a modulator ---
http://www.ebay.com/itm/DUKANE-35-Amp-Solid-State-Amplifier-PA-1A1335A-2-Microphones-Guaranteed-/261015395859?pt=Vintage_Electronics_R2&hash=item3cc5bade13Amps like this tend to go cheap 'cuz they're not 100 Watts or better, but 35 Watts RMS is more than enough to drive the EICO to full modulation and it has mic pre-amps, tone controls plus a transformer isolated output. With an amp like this the 35 Watt rating is real and not BS "music power" so there's some reserve should you want to add a compressor circuit.
