G'Day Paul,
Difficult one to give any sort of exact answer, the way I try to look at it is rather than carry a heap of tools just in case try to work out the most likely thing on your bike that could need attention, usually magneto or carby, so........
Spare plugs and a plug spanner
Whatever you need to clean and adjust the points (piece of emery paper and ignition spanners and feeler gauge)
Ditto carby, enough tools to remove and dismantle the carby.
A couple of small shifters and a screwdriver.
You have to take into consideration how far you are going, if you are only riding locally then just the very basics is enough, if you are planning a trip away then more is needed such as being able to adjust the chain and change tyres etc.
Most important of all, MOBILE PHONE !