Thank you all for your comments. In fact, the voltage developed by a magneto is proportional to mainly three variables:

1)- The magnetic field strength (which is proportional to the magneto gap).
2)- The rate of change of the magnetic field over the magneto coil (which is proportional to the prop RPM).
3)- The number of wire winding turns inside the magneto coil.

There is a saturation point above which no increase in voltage is produce. The other thing to consider, is that you might have a very large voltage, but if the magneto coil is wind with a very thin conductor, then it's internal resistence wiuld be so large that it would not be able to produce enough "current" and the spark would be weak. In general the greater the voltage, the lower the current for a certain amount of power. I think the magneto on this engine produces weaker spark than that of Zenoahs for instance,

I used to have a Zenoah engine and it started very well using the choke as previously mentioned and the spring starter. I am being able to start the US-41 with the spring starter, but I definitely have to dry the spark plug after I manually prime the engine, otherwise it won't start.

Could you explain in more detail your choke design using a piece of tin?

Thanks.