I doubt you will need the transformer at all. 500R DC resistance (post #1) implies the impedance is higher than that so you should be OK. The matching isn't at all important anyway, you can adjust the mic level control to compensate for almost any input.
I might have the service manual for the FT-101 gathering dust somewhere. I have several Yaesu manuals but they have been stored away for many years and I can't remember which models they were for. If I have it you are welcome to it, I don't think I will be needing it in the future, my days of repairing transceivers are long gone.
Hint: add a resistor of 10R or so in series with the PTT switch, it shouldn't stop the relay working but it will help to protect the switch from directly discharging C54.
Brian.