Is it necessary to protect old devices against "new" 230V?
From a safety point of view, it's highly unlikely you need to change anything unless the original product was dangerous to start with. As early as 1910 might have had a different/even wider tolerance but before the 230 V unified Europe, 220 and 240 V was +-10 % and "new" 230 V is +-6 % so you will still be within the tolerance band.

From a performance point of view, yes, it may be completely linear relation between speed and input voltage. Simplest but heaviest solution would be a transformer, either straight 230 V -> 220 V, an autotransformer/variac or if you find one multitapped primary with both 220 and 230 V with suitable VA rating, you are good to go.