How hot is it normal for a 45 watt MagSafe Adapter to get during use

90˚c is typical for that system under high load; anything > than 95˚c is when you might want to be concerned. If the CPU gets too hot, or starts to overheat, the system (typically) will clock down the CPU to prevent overheating. The fans shouldn't wear out, so I don't really see any point in trying to replace what's not broken. Intel Mac's run hot, and the fact that the only fan is an output fan doesn't help with heat dissipation.

Something you could do to help alleviate some of the heat is to download smcFanControl (free) and adjust your fans RPM. Instead of the standard 1800 RPM set it at 3500-3600 RPM, which should help cool down the temp possibly 2-10˚c. You might notice more noise from the higher RPM, although you shouldn't worry about it — MacBooks fans generally max out at around 6,000 RPM, and I've cranked one up to 8,700 RPM before.

Your computer is designed to run all 4 cores and generate heat like a 60 w lightbulb would. Internally, the CPU doesn't get as hot as a filament does - but it does approach the boiling point of water before it will slow itself down to avoid overheating.

This control is built into the chips themselves so even if your fans stop working, the machine should run as fast as it can given the amount of heat that escapes through the warm aluminum frame. The heat that you have noticed may simply be the system working as designed with passive cooling assisting the fans.

Here is an article explaining a bit on how you can measure the internal temperatures, how the loading of work on the CPU spins up (and even sound recordings of) the fans:

• http://www.anandtech.com/show/6023/the-nextgen-macbook-pro-with-retina-display-review/12

My guess is your mac is running normal as it can be quite uncomfortably hot and still operating as designed. However, if you are worried, you should call AppleCare and ask them for assistance in determining if it's running safely. Rather than trust that our understanding of how hot your Mac is since you could be someone who is underestimating danger, or another that over estimates it and I'd hate to not have you get a second opinion or know exactly how hot the internals and case are. Consulting the manufacturer would be a great next step for you.