The G3 should not run hot, have you used compressed air to clean out the powersupply and case to enable the fans to work properly?
My guess is DIY but have a look at xlr8yourmac.com. As I recall the heatsink was well clear of any obstructions in the case so you could put a fan on top of the heatsink and run the fan from 12V.
If it is crowded you can buy a harddisk cooling fan, they have one or two fans blowing air straight up in the case that will help cooling the CPU.