How to add hybrid technology to a 4-stroke engine motorcycle?

Converting existing vehicle to a hybrid is a difficult proposition, and even more so with motorcycle due to limited space allowance.

Motor and Drive-train

In a small motorcycle like that one would have to go with the wheel hub motor like that:

Those could be fairly expensive, plus there is an additional cost of re-spoking the rim with this hub, and converting to a disc brake (since yours are drum brakes).

More-over, this being a hybrid design, there is considerable engineering challenge of coupling and de-coupling this motor from the rest of drive-train or the motor. Possibly, there could be a clutch that disengages the hub from the rear sprocket, but then it would compromise the rigidity of the unit, as the sprocket must be securely attached to the hub to transfer power, and I am not well versed in mechanical engineering to know whether there are clutches available that would accomplish that. Again, that would also add bulk, and unsprung weight to the rear wheel.

Alternately, the same hub could be put onto a front wheel, and then to operate such hybrid system, one would have to manually put the gearbox in neutral before operating the electric motor. The latter requirement is a given, as the automatic switching between two modes would be prohibitively complex and expensive on such a small machine.

Manual switching, however, gives flexibility of using the electric mode in different ways: either slow-speed parking-lot cruising, or using the electric motor to maintain higher speed while the petrol engine gets the bike up to that speed (similar to the Porsche hybrid system). Either way, I would stuck with petrol propulsion for hard acceleration or heavy loads.

Battery and Electronics

Given the limitations of space and weight, one would have to go with high energy density battery such as Lithium-ion, but that again would add cost. Theoretically, one could add couple automotive batteries to the rack around the rear wheel, but those are heavy and would significantly cut into the useful load of the vehicle.

Then there are electronic controllers and regulators to run this system. If we settle on the manual switching between the modes of propulsion, it significantly simplifies the electronics, mechanics and controls of the system, but it still requires additional electronic equipment and wiring that needs to be placed somewhere. As an example solution to this problem, new batteries could take over the duties from the small on-board battery, which can be taken out, and its space could be used for the controllers and regulators.

Conclusion

While this would be curious engineering exercise (and, possibly, good university thesis for some electrical engineering graduates), it is, in my under-informed opinion, a futile one. It is too complex, and too expensive to justify such conversion. You already have good, reliable machine that is low on the fuel consumption and decent with emissions even as it is a carburetted one. I have similar bike (Suzuki TU250X/ST250E/Volty), which has the same motor with larger cylinder, and EFI, and it gets 2.7‒2.8 l/100 km in fuel consumption when ridden conservatively, and it is a lot better than the car as personal mode of transportation.

If you are interested in electric transportation, you would be better served by converting a bicycle to electric, or buying old broken-down small motorcycle and converting it into fully-electric commuter. There are plenty information about such conversions, and it is a lot cheaper than trying to put together a hybrid.