If you are at 40kV or 65kV, is that a problem, to have poor regulation?
I think the hard part will be controlling the output voltage - this runs off 3.7VDC but rated to 12VDC power (with some smoke me thinks). I would be tempted to instead use an IC SMPS/oscillator and measure the output voltage to regulate things, but HV gigaohm resistors are expensive and huge.
Wonder if 15kV is absolute max. this can generate, it seems to be the transformer's insulation limit.
So you could take <10kV and a 6-stage multiplier. Sometimes it's cheaper to use more multiplier stages than the higher voltage rated parts.
The
2CL77 HV rectifier diodes look good, 20kV 5mA 100nsec. and cheap on Ali $0.25 each. 20kV 10nF disc caps are $1.12 ea.