200kHz Ultrasonic Piezo Transformer
I'd start with using a ferrite material like 3F3 from Ferroxcube - it'll have low losses at 200kHz: -



The dotted line in the graph (largely) represents the core losses and, at 200 kHz, it is quite low compared to the normal permeability of the material.

I'd probably go for an RM type pot core such as an RM8 (available with 3F3 material) and I'd gap it it to reduce efects of saturation and inductance drift with temperature. The part is available pre-gapped and has a Ferroxcube part number of RM8/I-3F3-A400 - this has an \$A_L\$ value of 400 nH/\$\sqrt{Hz}\$. If you have 5 turns on the primary its inductance will be 10 uH.

The secondary would be of course 50 turns. The bobbin part number is CPV-RM8/I-1S-12PD - it should have enough room to put the windings in. You'll need two clips to hold the ferrite parts together (CLI/P-RM8/I).

If you refer the transducer capacitance back to the primary you'll be seeing about 100nF because of the square of the turns ratio and this will look like a primary load of 10 uH in parallel with 100nF (plus a resistor that represents the acoustic power output from the piezo. This will tend to resonate at 159kHz so you might want to reduce the primary turns to about 4 to reduce the inductance.

I'm not sure whether the piezo transducer you have is very resonant at 200kHz or not and this could make a big difference - I'm assuming that it is not naturally resonant at 200kHz but if it is you might need to experiment with primary and secondary turns. Alternatively, if you have a good model of the electrical characteristic try simulating the effects it might have - you won't need to worry about simulating the 3F3 material - just use two coupled coils at about 97% coupling and play with primary inductance values.

That's how I'd do it anyway.