Try RS/Farnell catalogues (both online i think). If you cant order yourseld (company dont have an account etc etc) drop me a PM and i should be able to assist.
It depends, and now, to be honest, it looks less good.... I'll explain...
The Voltmeter you couldnt reduce the voltage as that would cause incorrect readings.
If the Oil Pressure Guage is mechanical, the voltage would be just for the lighting circuit, but if its electronic, you need a new sender.
Your temperature sender unit will be 12v, so I think that would have to be changed!
The tacho would work with transformed voltage
The ignition switch (as its a switch) will work at 12v.
So I've talked myself and you out of that, I have this as a suggestion. Take your panel and measure the apatures of the guages and switches, post them here or PM me with the dimentions and I will find you top class replacements for less than Perkins can do them.
Guages is guages, and with my passion for things 4 wheeled and fast, I have bought my fair share.
I'd hold off of the Ebay bidding on this one if I were you.