What could cause an old electric oven to run hot even when adjusted to the lowest setting?
You could try to replace the thermostat if you can find a suitable replacement.  But there may be a very low-tech solution.   If the oven temp is consistently 100 higher across the entire range of the oven knob, you can either repaint the knob to shift all the numbers over or you can paint an indicator arrow on the oven surface next to the knob but shifted over.  So, for example, you can set the knob to 250 then paint an indicator next to the knob at 350.  The drawback to this is that someone who is unfamiliar with your oven may be confused and could leave the oven on when they think they are turning it off.  To solve that problem, paint a separate set of indicators on both the knob and the oven surface to positively indicate when the ofen is off.