/\ That is possible.
However you will need a considerable number of small parts to make the engine truly variable speed, the easiest option is usually to find a scrap engine with these parts already present on it, but even after this you may still have to buy another set of governor springs, as there is a multitude of choices depending upon what speed range you want the engine to run.
Sleeman and Hawken in the UK will have these items, but they will not be cheap.
A first step would be to located an instruction manual for your engine, so you can see what this involves.
Also bear in mind, although your engine is fixed speed, it is possible to adjust the engine speed +/- 5% by adjusting the tension on the governor spring (the adjusting screw is on the top of the timing cover, just underneath the fuel tank, screw out to increase / screw into to reduce engine speed)