There used to be a limit (think it was 200cc) below which the machine could not be used for an IAM test. This was set some time ago and was based on the machine's ability to cope with Motorway Speeds with all manner of riders.
This was reviewed a few years ago due to the technological improvements and increased power available from the smaller capacity machines.
However, as JamesH mentions, the opportunities to overtake etc. are increased when one uses a more powerful machine. Something which gets up to motorway speed reasonably quickly and can maintain it should suffice, but something which is capable of much more does give the rider a considerable safety margin to work with and will not strain the machine.
Again per the earlier post... get your DSA and log up a few hours then contact your local IAM / RoSPA Group and progress to the Advanced style.
...and watch out for other road users - very few actually see motorcycles