can i remove the rectifier and use a 12v dc input? Not necessary but i have a 12vdc 1200ma supply available.
Can i replace the transformer with something more readily available?
Sure. You might requre more current than your PS supplies though.
I doubt that would be a problem. Keep an eye on inductive spikes coming from the transformer primary.
If the specific transformer mentioned in the schematic is not available to you, any transformer with the specifications primary 117VAC, secondary 6.3VAC CT (center-tapped) should work.
In this case you might have to choose a different drive frequency in order to operate at the sweet spot of the transformer: "[...] I found in driving a transformer as is done in this project that the overall performance varied significantly as I varied the drive frequency. In a series of experiments I loaded the transformer directly with a variable resistor (no rectifier used), measured the AC voltage across this resistor, measured the DC current into the transformer, and varied the drive frequency. I looked for a frequency where the output voltage was highest and the input current was lowest. [...] 'ripple' is roughly +/-25V for output voltage >200V. This ripple isn't sinusoidal at all. It is basically a spike then monotonically decays until the next spike."
You found a powerful circuit. Its high power makes it complicated and expensive.
But your geiger tube does not need high power. It needs a simple little (cheap) circuit.
This is an OLD Posting on this subject, But this info might help others in the future.
And as Stated by Audioguru, Geiger-muller tubes only require Low current.
Just in the Low Micro-Amps.