I'm not so sure that it would stand much over-volting - if we are talking about the Suzuki motor? It uses forced air cooling which would suggest to me that it runs rather hot if on full for any length of time. As we know: Heating is proportional to the square of the current, and current is directly proportional to voltage . . . . well, up the voltage even just a little and something that runs hot will run even hotter - OK so as the temperature increases then the current will fall . . . but line voltage of 13.6 is probably the most that this little motor could ever take IMHO
As an aside, I found a Mercedes (300 something I think) core that fitted with very little fettling, into the housing, and even with the same agricultural blower, the heat output was about 50% more - very pleased. All the best, Lady Smith