Hi Agustín,
First question: See what you can find, if it has two gates it will do.
BF961's and BF981's will probably work even better
than the 40673's.
Second question: None of the toroids in your junkbox are suitable, that's
why it's called the junkbox.
The T-cores are metal powdered cores, the FT-cores
are ferrite cores.
**broken link removed**
Third question: You don't actually need the audio amplifier, get the
circuit working and decide later if you want to add one.
This is what a T50-2 looks like.
on1aag.