Hi Burras,
Well you've got a few options really and it's all dependant on the location where you'll be recording.
Option 1
If you're able to run a power lead to the location of the bike, you could get yourself an SM57(or whatever mic you choose) and a small mixer like this one.
http://www.guitarampkeyboard.com/en/...enyx-502/70279
That would allow you to plug one mic in, adjust the levels(i.e. so it's not peaking) and send the source out to your minidisc.
Option 2
Buy something like an SM57 and plug it straight into your minidisc with a relevant cable(i.e. XLR -> Jack). I tried that this morning with my minidisc recorder and my SM57, it was acceptable but I'm not sure how it's going to handle high volumes.
This solution would be portable, but probably won't give you the results you're after in terms of quality.
Option 3
You could also consider something like this:
http://www.guitarampkeyboard.com/en/zoom-/71334
I've never used one myself(so I can't guarantee it's going to suit you 100%) but it's portable and has in-built mics. It's probably a little overkill for what you need but it's an option.
Like I said, it's going to be a balancing act between portability and quality. If you're in a controlled location(i.e. without other bikes whizzing around) you may want to consider a condenser microphone that will pick up more of the surrounding noise.
HTH