Powering an old two box metal detector
You will probably find it eats 1.5V batteries but the 45V current draw is quite low... 

As for approximating the 22.5/45V voltages; I recommend erring on the low side rather than going over-voltage. The glassfets will take the over-voltage quite happily, but 50-year old capacitors, probably not... 

And HT batteries were commonly used down to a fraction of their nominal voltage, so a stiff supply from rechargeables at 90% of rated voltage should be fine.

An alternative to rechargeables for the 45V supply would be a stack of cheap eBay lithium coin cells.

Also be aware that old battery equipment often used a quite alien design pattern to modern designers : there was no direct DC path from HT to earth; therefore the HT battery was permanently connected, not switched. The only current path from HT was through the glassfets, so switching their heaters off reduced the HT current to 0. (Check this with the heaters off; any leakage may indicate a faulty capacitor.)