Well... I guess it's not extremely difficult to try.
Though Powermacs G5 are another world (yup!) compared with G4 (and with anything else, I suppose..) I'd rather sure to perform all operations.
I have to say I've dismounted two G5 dual 2.7 (one of them is a skeleton now!) to recover parts of one and to clean and check for leaking of the other. You need the right tools, spare parts, ASD disc for calibrate cpu and fans and a whole set of patience. Obviously, reading carefully every sort of documentation that it's possible to find online and go step by step.
Interesting thread about repairing a LCS of a dual 2.7:
http://forums.macrumors.com/showthread.php?t=1109014
Pumps for liquid cooled system (just in case...):
Swiftech 12V DC Water Cooling Pump - MCP350
Viton O-Rings to replace:
Viton O-Rings
Video about how to calibrate a G5:
Powermac G5 fan calibration
and so on.
Yes, this is not an easy operation but I would (and probabily will) do to my G5.