it is possible to mount a faster CPU, however there are a couple of things you have to consider.
1. The passive heatsink might not be big enough to cool the faster cpu efficiently. So, it might be necessary to add a blower.
2. The power supply of the IA-1 was designed to supply the IA-1 as is. Normally, it should have some reserve for the USB ports. A USB port should be able to deliver 0.5 Amps on 5V. With a faster CPU, your power supply might come into problems, especially if you add a couple of USB devices (Like memory sticks and wifi sticks)
3. The IA-1 bus speed is 66Mhz. If you install your faster CPU, it will probably require a bus speed of 100Mhz if you want the CPU to operate at it's normal speed. In my IA-1, I have two small SMD resistors (0201 size) which should be soldered on another place to change the bus speed. (At least, thats what I think according to what's printed on the PCB.) It is not a easy job to do this without the proper equipment.
I have to admit that my answer is mostly based on other articles in this threat. I haven't tried a cpu update myself yet. So, this is not really a answer to your question, but it might be usefull explaining the problems you can have (hopefully not).