Hello,
I would check all of the interlock switches (i.e. those that are necessary to be closed before the ignition will activate). There is usually one under the seat (to make sure the driver is in position prior to starting), one on the brake and one on the drive direction and blade control (i.e. to prevent reverse motion, whilst the blades are activated).
Also, a friend of mine recently had trouble because he changed the earth/-ve lead (between the battery and the starter motor), for another which was not of sufficient quality/ current carrying capacity (having too much electrical resistance) and would not allow sufficient current through the starter....
I hope this helps with your problem-solving
Regards
Ricehofen, November 2011