voodoo 2's don't even need heat sinks let alone more case fans , they'll but fine in that case as is.

You could drill holes for a couple 60mm exhaust fans on the back of that beige case it looks like. You can spend $30 or so on some nice ones and be done with it. Love the look of that case btw.

A 120mm would fit for the rear.
Depending on how the design is, cut the back and hammer it flat. Then hammer a new 90 deg edge and rivet it back on. Many ways to do it with scrap metal and rivets.

Yeah those Aopens are not the best for cooling. Maybe whack a 12cm on the front, it's been done before...
http://www.dansdata.com/hx45fan.htm

Some options that require no physical modifications to the case whatsoever would be fans on the front that fit into one of the unused 5.25 drive bays, plus an exhaust fan that mounts in one of the unused expansion slots, as shown below.

One downside to this I learned from experience is that the fans in the drive bay can be quite annoying, because small size means faster speed to move the same amount of air...multiplied by however many fans are in the fixture. But, like I said, at least it would require no actual modifications to the case. If you have the size adapter brackets to do so, you could even mount the hard drive in that same bay and you'll get the extra benefit of the bay fans helping keep the drive cool.