Bought this on Amazon: 

Toroid Core FT240-31 Ferrite

3 inch diameter
0.25 width

Type 77 core material is better, so bought some of them as well, type FT140-77, from Amazon. OD=35.55 mm, ID=23mm. 

According to online inductance calculator at https://coil32.net/online-calculators/ferrite-torroid-calculator.html, inductance for N turns of AWG 20 wire is 2 uH X N^2 of inductance.
So 10 turns on the primary would have an inductance of 200 uH. 
(Permeability of the type 77 ferrite is 2000).  
resonance with 600 pF piezo has omega=1/sqrt(LC) = 1/sqrt(600e-12*200E-6) so f= 400 kHz. Impedance at 100 kHz is omega*L = 2* pi* 1e5*200e-6 = 125 Ohms. 
If we have 1:10 ratio of primary to secondary turns, then 

From Wikipedia: 

 

So if we want to boost applied voltage to piezo from 10V to 100V, we need a turns ratio of 10 turns on secondary for every turn on primary. 
Impedance presented to drive circuit then looks 100X higher.

Our fast power amp has output impedance of Z=V/I = 10V/6A = 1.6 Ohms.