Brian,
why not post this question (as the subject line clearly shows) in the ... Electronic section of this forum? People do look there you know.
You do not provide enough info. The insulated stranded wire which is included with the decoders is usually 30 gauge (AWG). You don't say what gauge is your magnet wire. Magnet wire comes in many sizes (gauges). Depending on its gauge and length, you might only need to use a single magnet wire.
In practice (without doing any serious calculations, just going by experience), I often use magnet wire (usually 36 gauge or even smaller) for hooking up LED headlights.
Kato N scale motors (as indicated on the box) usually use less than 200mA of current. That is headlights and motor. That is probably while the loco is slipping. In a stall, the motor would consume more current.
There are online charts showing the current carrying capacity (and resistance per foot) for wire. Here is one such chart:
http://www.powerstream.com/Wire_Size.htmOnce you know the gauge of your wire (of you have a micrometer, make sure to measure bare wire (with the enamel stripped first), and you know the load (like 250mA for a motor or 5-20mA for LED headlight), you can figure out what gauge wire is safe for your application.