You can use some newer power supply with 20+4Pin the 4Pin brake away from the 20Pin but that not the problem the older motherboard use much higher amps/watts on the 3.3v/5v rail.
As for higher 12V and lower 3.3v/5v
There all same on all ATX up to ATX 2.3 when it come to voltage, The real problem start ATX 2.0 with amps/watts on 3.3v and 5v rails was significantly reduced how ever not all power supply are that way.
How ever if have any ISA in use you need find an older ATX12V 1.2 spec Pin 20 (formerly −5V, white wire) is absent in current power supplies; it was optional in ATX and ATX12V ver. 1.2, and deleted as of ver. 1.3.
How ever in you case you need Original ATX spec up to ATX12V 1.3 or look really hard you may find same ATX12V 2.01 which mainly need be designing the Intel Pentium 4/AMD K8
Keep in mind when they say higher 12V and lower 3.3v/5v there really ref to amps/watts.
You want min 3.3v 28amp / 5v 35amp or higher ampshttp://en.wikipedia.org/wiki/ATX#Interchanging_old.2Fnew_systems_with_old.2Fnew_PSUs
Go to newegg look for Linkworld LPG8