Everything sounds like it should be possible but I can't say for sure without knowing the system in detail.
You can go ahead and buy an MPGuino and start experimenting. If you know anyone with an oscilloscope you can make some measurements to see for sure if you have signals you can work with. Or you can buy an service manual for the bike and there should be sufficient detail in the manual to tell for sure if the MPGuino would work.
A manual will have details of the speedometer signal if there is one. Variations of the bicycle magnet approach work fine if you need to create your own speed signal.
I did look around a bit and was not able to find a manual I could download but they could be out there. I found a link for a printed manual and I'm sure there are plenty of other sources.
Honda CBF 125 M Workshop Manual
Mike