From memory the main differences are:
The engine on the Mk1 has different heads, barrels and pistons. The flywheel/clutch assembly only has 3 pins through the flywheel and also less rivets in the centre boss of the clutch plate.
The gearbox has a slightly different shaped main casing and different selector mechanism.
The mudguards and rear numberplate are peculiar to the Mk1.
The Mk1 toolbox has flat sides whereas the Mk3 is slightly domed.
The Mk1 headlight has a flat glass.
The Mk1 had a straight pull twistgrip and carbs with covered bellmouths with vent holes around the sides.
There may also be other detail differences.