36 volts would very likely be too much for a 24volt controller so you would need to change the controller. The heinzmann motor I had was a brushed motor which would go faster with the higher voltage, the danger would be when you try and go up a hill with the higher voltage it might overheat.
I have run brushed dc motors on twice their normal voltage before without blowing them up but they do get hotter.
Depends how gutted you would be if you blow it up really!