Two differences I can think of.
The 168 tooth flywheel is probably for a 12 volt starter and may well be larger for a larger clitch disc, 9-1/2" to 10". The 153 tooth flywheel may be for a 6 volt flywheel and the clutch be in the 9" range. The 153 tooth would take the 6 volt starter, but could still be a 12 volt system. You can spin a 6 volt starter with a 12 volt system, you just can't stay on it too long or it will get overheated.
Sorry!
