You will still end up spending twice the purchase price to mod one, however.
The diagram is accurate, and the system works. When the boost relay is activated, the additional 12 volts is added to the existing 36 volts, and this goes right to the motor, bypassing the solid stand controller. The power diode prevents current from feeding back into the controller. The diode is important. A third relay could be used in its place, but I suspect it would not switch fast enough to avoid damaging the controller, hence the diode. Plus the diode is a little cheaper.
However, putting 48 Volts to a 36 Volt motor might shorten its service life - although some old-timers say these motors can handle the voltage. It also means your brakes will have to work harder to stop. Finally, if you "floor" the accelerator from a standstill, the buggy will lurch forward and that can be kind of dangerous (it is unexpected) and also put stress on the pinion gear in the rear axle. But the fellow who came up with this puts 72 volts to his buggy (for a total of 108 Volts!), so at least we are not being that extreme!