I concur with ThorRush, they may not have the technology to upgrade.
Also, most of Urgot's mechanical parts assist in his survival, any serious modifications could be potentially risky, seeming as they only barely managed to resurrect him in the first place.
Viktor would be barred out, as Urgot's ressurection was Pididily's brainchild, who would likely take offense and pull out from any further work on Noxian forces if he was upgraded with Viktor's tech. Viktor would attempt to do more than simply improve his durability anyway, it's like the difference between installing a pacemaker (or something along those lines) and installing a full mechanical heart. The problem is that Pididily built the mechanical body to keep what was left of Urgot alive, Viktor would simply do away with that and replace the bio-matter completely, and I'm not sure if the Noxian's are completely open to that.