1 - A Via C3 clocked at 1Ghz is about the same as a P-3 at the same clock speed. The appeal of the Via C-series CPU is their very low current draw, which comes at the expense of computing horsepower.
No, you can't upgrade the CPU on a Via embedded motherboard. tHe CPUs are soldered to the board.
2 - We're not going to hunt down motherboards to mate with your CPU. Rather than apporach it from the standpoint of "I have this CPU. What can I use with it?", I suggest apporaching it from the perspective of "What will minimize current draw, fit in my vehicle, not overheat, and stay within my budget? What hardware do I already have that meets this critera?"
Many times it is far more cost-effective to scrap hardware you have in favor of new hardware that will make the project work as you envision.
Whatever the case, I welcome you to our maddening hobby. I myself encountered a dead motherboard today. Maddening. But fun.
And check out the FAQs. They're chock full of information for you, just waiting to be located and put to use.