Powerbook to MacMini conversion

some of those old powerbooks got really hot even when new. but from your post it sounds like you're more annoyed with the sound of the fan and not the temperature of the machine. does your powerbook use thermal paste anywhere in there? it probably needs replacing. and blow out all the dust inside while you have it open.

 

Check where the temperature is coming from:
"HardwareMonitor"
https://ssl.webpack.de/bresink.eu/Downloads/HardwareMonitor-499Legacy.dmg

Change the threshold for CPU/GPU/HDD - so you won't have to fiddle around with the hardware:
"G4FanControl"
https://www.andreafabrizi.it/?g4fancontrol

Maybe replace the thermal-paste/pads of CPU/GPU, but be careful when removing the two screws, which are enforced with springs! You'll have to press the heatsink down until both screws are removed, then carefully lift the heatsink up keeping it horzontally to avoid a screw from bending and breaking off the mainboard. (http://forums.macrumors.com/threads/the-return-of-an-old-friend.1931737/#post-22384355)
To keep the Powerbook cool I'd recommend a metal stand for better heat conduction - but I guess it's a matter of the fan's threshold. My PowerBook's fans did run without a stop after I replaced the HDD with an mSATA, since the temperature of the mSATA is much higher than the treshold level for normal mechanical HDDs and only G4FanControl finally made the fans stop.