Powering an old TI Silent 707 with a missing external AC/AC adapter
/ WATTS 150 W  120V. Standard IEC Plug

Is this your model?  I suppose the isolation transformer is necessary for preventing 0Vdc from being offset from Neutral or miswired Hot to neutral swaps.
Yes I would use a 65W min 16V universal power supply. It has a non isolated DC to Dc converter, so other than possible EMI crosstalk, and charging up 3.3mF cap on startup before turning ON pwr switch,  it should work...
The 150W AC rating is generous for power on surge and charger losses.